--- a/js/src/config/static-checking.js
+++ b/js/src/config/static-checking.js
@@ -58,17 +58,17 @@ function hasAttribute(c, attrname)
 }
 
 // This is useful for detecting method overrides
 function signaturesMatch(m1, m2)
 {
   if (m1.shortName != m2.shortName)
     return false;
 
-  if (m1.isVirtual != m2.isVirtual)
+  if ((!!m1.isVirtual) != (!!m2.isVirtual))
     return false;
   
   if (m1.isStatic != m2.isStatic)
     return false;
   
   let p1 = m1.type.parameters;
   let p2 = m2.type.parameters;