diff --git a/panda/src/pgraph/shaderInput.I b/panda/src/pgraph/shaderInput.I
index b8e182421f..d0a0e79fbb 100644
--- a/panda/src/pgraph/shaderInput.I
+++ b/panda/src/pgraph/shaderInput.I
@@ -490,7 +490,19 @@ operator < (const ShaderInput &other) const {
     return false;
 
   case M_vector:
-    return _stored_vector < other._stored_vector;
+    if (_stored_vector[0] != other._stored_vector[0]) {
+      return (_stored_vector[0] < other._stored_vector[0]) ? -1 : 1;
+    }
+    if (_stored_vector[1] != other._stored_vector[1]) {
+      return (_stored_vector[1] < other._stored_vector[1]) ? -1 : 1;
+    }
+    if (_stored_vector[2] != other._stored_vector[2]) {
+      return (_stored_vector[2] < other._stored_vector[2]) ? -1 : 1;
+    }
+    if (_stored_vector[3] != other._stored_vector[3]) {
+      return (_stored_vector[3] < other._stored_vector[3]) ? -1 : 1;
+    }
+    return 0;
 
   case M_numeric:
     return _stored_ptr._ptr < other._stored_ptr._ptr;