diff --git a/stepmania/src/ActorCommands.cpp b/stepmania/src/ActorCommands.cpp
index 959d97ec9d..f6baadcc36 100644
--- a/stepmania/src/ActorCommands.cpp
+++ b/stepmania/src/ActorCommands.cpp
@@ -17,7 +17,9 @@ void IncorrectActorParametersWarning( const ParsedCommand &command, int iMaxInde
 void ParsedCommandToken::Set( const CString &sParam )
 {
 	s = sParam;
-	f = Lua::RunExpressionF( sParam );
+	CString s = sParam;
+	Lua::PrepareExpression( s );	// strip invalid chars
+	f = Lua::RunExpressionF( s );
 	bColorIsValid = c.FromString( sParam );
 }
 
diff --git a/stepmania/src/LuaHelpers.cpp b/stepmania/src/LuaHelpers.cpp
index 772548252f..1bb4d53c35 100644
--- a/stepmania/src/LuaHelpers.cpp
+++ b/stepmania/src/LuaHelpers.cpp
@@ -184,6 +184,17 @@ void Lua::UpdateGlobals()
 	lua_setglobal( L, "SCREEN_CENTER_Y" );
 }
 
+void Lua::PrepareExpression( CString &sInOut )
+{
+	// HACK: Many metrics have "//" comments that Lua fails to parse.
+	// Replace them with Lua-style comments.
+	sInOut.Replace( "//", "--" );
+
+	// Remove leading +, eg. "+50"; Lua doesn't handle that.
+	if( sInOut.size() >= 1 && sInOut[0] == '+' )
+		sInOut.erase( 0, 1 );
+}
+
 void RunExpression( const CString &str )
 {
 	LoadFromString( L, "return " + str );
diff --git a/stepmania/src/LuaHelpers.h b/stepmania/src/LuaHelpers.h
index 7860c2e30c..bafc053c42 100644
--- a/stepmania/src/LuaHelpers.h
+++ b/stepmania/src/LuaHelpers.h
@@ -4,6 +4,7 @@
 struct lua_State;
 namespace Lua
 {
+	void PrepareExpression( CString &sInOut );	// strip "//" comments and "+"
 	bool RunExpressionB( const CString &str );
 	float RunExpressionF( const CString &str );
 
diff --git a/stepmania/src/ThemeManager.cpp b/stepmania/src/ThemeManager.cpp
index be7aa61b30..e9c1983fce 100644
--- a/stepmania/src/ThemeManager.cpp
+++ b/stepmania/src/ThemeManager.cpp
@@ -615,13 +615,8 @@ int ThemeManager::GetMetricI( CString sClassName, CString sValueName )
 float ThemeManager::GetMetricF( CString sClassName, CString sValueName )
 {
 	CString str = GetMetricRaw( sClassName,sValueName );
-	// HACK: Many metrics have "//" comments that Lua fails to parse.
-	// Replace them with Lua-style comments.
-	str.Replace( "//", "--" );
 
-	// Remove leading +, eg. "+50"; Lua doesn't handle that.
-	if( str.size() >= 1 && str[0] == '+' )
-		str.erase( 0, 1 );
+	Lua::PrepareExpression( str );
 
 	return Lua::RunExpressionF( str );
 }
@@ -634,9 +629,9 @@ bool ThemeManager::GetMetricB( CString sClassName, CString sValueName )
 		return false; /* optimization */
 	if( str == "1" )
 		return true; /* optimization */
-	// HACK: Many metrics have "//" comments that Lua fails to parse.
-	// Replace them with Lua-style comments.
-	str.Replace( "//", "--" );
+
+	Lua::PrepareExpression( str );
+	
 	return Lua::RunExpressionB( str );
 }