diff --git a/dtool/src/cppparser/cppBison.yxx b/dtool/src/cppparser/cppBison.yxx
index a9ac210d46..7a9797148e 100644
--- a/dtool/src/cppparser/cppBison.yxx
+++ b/dtool/src/cppparser/cppBison.yxx
@@ -1891,7 +1891,7 @@ instance_identifier_and_maybe_trailing_return_type:
   }
   $$ = $1;
 }
-        | instance_identifier ':' INTEGER
+        | instance_identifier ':' const_expr
 {
   // Bitfield definition.
   $1->_bit_width = $3;
diff --git a/dtool/src/cppparser/cppInstance.cxx b/dtool/src/cppparser/cppInstance.cxx
index cb59e9a048..3bb3bb725a 100644
--- a/dtool/src/cppparser/cppInstance.cxx
+++ b/dtool/src/cppparser/cppInstance.cxx
@@ -74,7 +74,17 @@ CPPInstance(CPPType *type, CPPInstanceIdentifier *ii, int storage_class,
   ii->_ident = nullptr;
   _storage_class = storage_class;
   _initializer = nullptr;
-  _bit_width = ii->_bit_width;
+
+  if (ii->_bit_width != nullptr) {
+    CPPExpression::Result result = ii->_bit_width->evaluate();
+    if (result._type != CPPExpression::RT_error) {
+      _bit_width = ii->_bit_width->evaluate().as_integer();
+    } else {
+      _bit_width = -1;
+    }
+  } else {
+    _bit_width = -1;
+  }
 
   CPPParameterList *params = ii->get_initializer();
   if (params != nullptr) {
diff --git a/dtool/src/cppparser/cppInstanceIdentifier.cxx b/dtool/src/cppparser/cppInstanceIdentifier.cxx
index 878d751285..891383cacc 100644
--- a/dtool/src/cppparser/cppInstanceIdentifier.cxx
+++ b/dtool/src/cppparser/cppInstanceIdentifier.cxx
@@ -82,7 +82,7 @@ initializer_type(CPPParameterList *params) {
 CPPInstanceIdentifier::
 CPPInstanceIdentifier(CPPIdentifier *ident) :
   _ident(ident),
-  _bit_width(-1),
+  _bit_width(nullptr),
   _packed(false) {
 }
 
diff --git a/dtool/src/cppparser/cppInstanceIdentifier.h b/dtool/src/cppparser/cppInstanceIdentifier.h
index 6a2951d9cb..9cf7a3edbe 100644
--- a/dtool/src/cppparser/cppInstanceIdentifier.h
+++ b/dtool/src/cppparser/cppInstanceIdentifier.h
@@ -86,8 +86,8 @@ public:
   typedef std::vector<Modifier> Modifiers;
   Modifiers _modifiers;
 
-  // If not -1, indicates a bitfield
-  int _bit_width;
+  // If not null, indicates a bitfield
+  CPPExpression *_bit_width;
 
   // Indicates a parameter pack
   bool _packed;