diff --git a/core/src/avm1.rs b/core/src/avm1.rs
index ee959f957..f074a7ac0 100644
--- a/core/src/avm1.rs
+++ b/core/src/avm1.rs
@@ -489,13 +489,14 @@ impl<'gc> Avm1<'gc> {
     pub fn current_register(&self, id: u8) -> Value<'gc> {
         if self
             .current_stack_frame()
-            .map(|sf| sf.read().has_local_registers())
+            .map(|sf| sf.read().has_local_register(id))
             .unwrap_or(false)
         {
             self.current_stack_frame()
                 .unwrap()
                 .read()
                 .local_register(id)
+                .unwrap_or(Value::Undefined)
         } else {
             self.registers
                 .get(id as usize)
@@ -515,7 +516,7 @@ impl<'gc> Avm1<'gc> {
     ) {
         if self
             .current_stack_frame()
-            .map(|sf| sf.read().has_local_registers())
+            .map(|sf| sf.read().has_local_register(id))
             .unwrap_or(false)
         {
             self.current_stack_frame()
diff --git a/core/src/avm1/activation.rs b/core/src/avm1/activation.rs
index 09c559c8e..c748a8250 100644
--- a/core/src/avm1/activation.rs
+++ b/core/src/avm1/activation.rs
@@ -41,6 +41,10 @@ impl<'gc> RegisterSet<'gc> {
     pub fn get_mut(&mut self, num: u8) -> Option<&mut Value<'gc>> {
         self.0.get_mut(num as usize)
     }
+
+    pub fn len(&self) -> u8 {
+        self.0.len() as u8
+    }
 }
 
 /// Represents a single activation of a given AVM1 function or keyframe.
@@ -275,25 +279,27 @@ impl<'gc> Activation<'gc> {
     pub fn this_cell(&self) -> GcCell<'gc, Object<'gc>> {
         self.this
     }
-    /// Returns true if this function was called with a local register set.
-    pub fn has_local_registers(&self) -> bool {
-        self.local_registers.is_some()
+
+    /// Returns true if this activation has a given local register ID.
+    pub fn has_local_register(&self, id: u8) -> bool {
+        self.local_registers
+            .map(|rs| id < rs.read().len() - 1)
+            .unwrap_or(false)
     }
 
     pub fn allocate_local_registers(&mut self, num: u8, mc: MutationContext<'gc, '_>) {
-        self.local_registers = Some(GcCell::allocate(mc, RegisterSet::new(num)));
+        self.local_registers = match num {
+            0 => None,
+            num => Some(GcCell::allocate(mc, RegisterSet::new(num))),
+        };
     }
 
     /// Retrieve a local register.
-    pub fn local_register(&self, id: u8) -> Value<'gc> {
+    pub fn local_register(&self, id: u8) -> Option<Value<'gc>> {
         if let Some(local_registers) = self.local_registers {
-            local_registers
-                .read()
-                .get(id)
-                .cloned()
-                .unwrap_or(Value::Undefined)
+            local_registers.read().get(id).cloned()
         } else {
-            Value::Undefined
+            None
         }
     }
 
diff --git a/core/src/avm1/function.rs b/core/src/avm1/function.rs
index 302e7ec7a..77e612660 100644
--- a/core/src/avm1/function.rs
+++ b/core/src/avm1/function.rs
@@ -29,11 +29,8 @@ pub struct Avm1Function<'gc> {
     name: Option<String>,
 
     /// The number of registers to allocate for this function's private register
-    /// set.
-    ///
-    /// If None, then no register set will be allocated and the preload options
-    /// have no effect.
-    register_count: Option<u8>,
+    /// set. Any register beyond this ID will be served from the global one.
+    register_count: u8,
 
     preload_parent: bool,
     preload_root: bool,
@@ -75,7 +72,7 @@ impl<'gc> Avm1Function<'gc> {
             swf_version,
             data: actions,
             name,
-            register_count: None,
+            register_count: 0,
             preload_parent: false,
             preload_root: false,
             suppress_super: false,
@@ -115,7 +112,7 @@ impl<'gc> Avm1Function<'gc> {
             swf_version,
             data: actions,
             name,
-            register_count: Some(swf_function.params.capacity() as u8),
+            register_count: swf_function.params.capacity() as u8,
             preload_parent: swf_function.preload_parent,
             preload_root: swf_function.preload_root,
             suppress_super: swf_function.suppress_super,
@@ -142,7 +139,7 @@ impl<'gc> Avm1Function<'gc> {
         self.scope
     }
 
-    pub fn register_count(&self) -> Option<u8> {
+    pub fn register_count(&self) -> u8 {
         self.register_count
     }
 }
@@ -215,52 +212,48 @@ impl<'gc> Executable<'gc> {
                     Some(argcell),
                 );
 
-                if let Some(register_count) = af.register_count() {
-                    frame.allocate_local_registers(register_count, ac.gc_context);
+                frame.allocate_local_registers(af.register_count(), ac.gc_context);
 
-                    let mut preload_r = 1;
+                let mut preload_r = 1;
 
-                    if af.preload_this {
-                        //TODO: What happens if you specify both suppress and
-                        //preload for this?
-                        frame.set_local_register(preload_r, Value::Object(this), ac.gc_context);
-                        preload_r += 1;
-                    }
+                if af.preload_this {
+                    //TODO: What happens if you specify both suppress and
+                    //preload for this?
+                    frame.set_local_register(preload_r, Value::Object(this), ac.gc_context);
+                    preload_r += 1;
+                }
 
-                    if af.preload_arguments {
-                        //TODO: What happens if you specify both suppress and
-                        //preload for arguments?
-                        frame.set_local_register(preload_r, Value::Object(argcell), ac.gc_context);
-                        preload_r += 1;
-                    }
+                if af.preload_arguments {
+                    //TODO: What happens if you specify both suppress and
+                    //preload for arguments?
+                    frame.set_local_register(preload_r, Value::Object(argcell), ac.gc_context);
+                    preload_r += 1;
+                }
 
-                    if af.preload_super {
-                        //TODO: super not implemented
-                        log::warn!(
-                            "Cannot preload super into register because it's not implemented"
-                        );
-                        //TODO: What happens if you specify both suppress and
-                        //preload for super?
-                        preload_r += 1;
-                    }
+                if af.preload_super {
+                    //TODO: super not implemented
+                    log::warn!("Cannot preload super into register because it's not implemented");
+                    //TODO: What happens if you specify both suppress and
+                    //preload for super?
+                    preload_r += 1;
+                }
 
-                    if af.preload_root {
-                        frame.set_local_register(preload_r, avm.root_object(ac), ac.gc_context);
-                        preload_r += 1;
-                    }
+                if af.preload_root {
+                    frame.set_local_register(preload_r, avm.root_object(ac), ac.gc_context);
+                    preload_r += 1;
+                }
 
-                    if af.preload_parent {
-                        frame.set_local_register(
-                            preload_r,
-                            child_scope.read().resolve("_parent", avm, ac, this),
-                            ac.gc_context,
-                        );
-                        preload_r += 1;
-                    }
+                if af.preload_parent {
+                    frame.set_local_register(
+                        preload_r,
+                        child_scope.read().resolve("_parent", avm, ac, this),
+                        ac.gc_context,
+                    );
+                    preload_r += 1;
+                }
 
-                    if af.preload_global {
-                        frame.set_local_register(preload_r, avm.global_object(ac), ac.gc_context);
-                    }
+                if af.preload_global {
+                    frame.set_local_register(preload_r, avm.global_object(ac), ac.gc_context);
                 }
 
                 //TODO: What happens if the argument registers clash with the
diff --git a/core/tests/regression_tests.rs b/core/tests/regression_tests.rs
index e7d3da9a9..097eaa56f 100644
--- a/core/tests/regression_tests.rs
+++ b/core/tests/regression_tests.rs
@@ -53,6 +53,7 @@ swf_tests! {
     (delete, "avm1/delete", 3),
     (timeline_function_def, "avm1/timeline_function_def", 3),
     (root_global_parent, "avm1/root_global_parent", 3),
+    (register_underflow, "avm1/register_underflow", 1),
 }
 
 /// Loads an SWF and runs it through the Ruffle core for a number of frames.
diff --git a/core/tests/swfs/avm1/register_underflow/output.txt b/core/tests/swfs/avm1/register_underflow/output.txt
new file mode 100644
index 000000000..52243d5d7
--- /dev/null
+++ b/core/tests/swfs/avm1/register_underflow/output.txt
@@ -0,0 +1,15 @@
+global reg 0: 0
+global reg 1: 1
+global reg 2: 2
+global reg 3: 3
+Function start f(a), RegisterCount = 3, a => register2
+Function reg 0: undefined
+Function reg 1: undefined
+Function reg 2: 66
+Function reg 3: 3
+Changing registers to 9.
+Function end
+global reg 0: 0
+global reg 1: 1
+global reg 2: 2
+global reg 3: 9
diff --git a/core/tests/swfs/avm1/register_underflow/test.swf b/core/tests/swfs/avm1/register_underflow/test.swf
new file mode 100644
index 000000000..6d5d28b02
Binary files /dev/null and b/core/tests/swfs/avm1/register_underflow/test.swf differ