Index: code/bmpman/bmpman.cpp
===================================================================
--- code/bmpman/bmpman.cpp	(revision 9593)
+++ code/bmpman/bmpman.cpp	(working copy)
@@ -1092,9 +1092,25 @@
 	return tidx;
 }
 
+/**
+ * Sanity check to ensure invalid bitmap handles are picked up early
+ *
+ * Some early known false or out of range handles (such as negative) are checked within
+ * an initial routine pass, followed by a more thorough check routine to ensure the  
+ * series of bitmaps within the bm_bitmaps[] structure have not become invalid.
+ *
+ * @param handle Bitmap handle to check for validity
+ * @return 1 for valid, 0 for invalid
+ */
 int bm_is_valid(int handle)
 {
+	Assertion( handle >= 0, "Out of range bitmap handle %d passed to bm_is_valid().\n", handle );
+    
+	// Ensure that certain known false or out of range handles are quickly returned as invalid,
+	// prior to utilising the handle in a way which leads to memory access outside bm_bitmaps[] 
 	if(!bm_inited) return 0;
+	if(handle < 0) return 0;
+    
 	return (bm_bitmaps[handle % MAX_BITMAPS].handle == handle);
 }