diff --git a/src/p_w32pe.cpp b/src/p_w32pe.cpp
index cc8b6e0b..e858fc0d 100644
--- a/src/p_w32pe.cpp
+++ b/src/p_w32pe.cpp
@@ -2388,6 +2388,7 @@ void PackW32Pe::rebuildImports(upx_byte *& extrainfo)
                 const unsigned ilen = strlen(++p) + 1;
                 if (inamespos)
                 {
+                    assert(oimpdlls != NULL); // FIXME
                     if (ptr_diff(importednames,oimpdlls) & 1)
                         importednames -= 1;
                     omemcpy(importednames + 2, p, ilen);
diff --git a/src/util.h b/src/util.h
index 2706b6e9..e65171b7 100644
--- a/src/util.h
+++ b/src/util.h
@@ -69,6 +69,7 @@ template <class T>
 inline int ptr_diff(const T *p1, const T *p2)
 {
     COMPILE_TIME_ASSERT(sizeof(T) == 1)
+    assert(p1 != NULL); assert(p2 != NULL);
     ptrdiff_t d = (const char*) p1 - (const char*) p2;
     assert((int)d == d);
     return (int) d;
@@ -76,6 +77,7 @@ inline int ptr_diff(const T *p1, const T *p2)
 #endif
 inline int ptr_diff(const void *p1, const void *p2)
 {
+    assert(p1 != NULL); assert(p2 != NULL);
     ptrdiff_t d = (const char*) p1 - (const char*) p2;
     assert((int)d == d);
     return (int) d;