diff --git a/panda/src/pnmimage/pnmimage_base.cxx b/panda/src/pnmimage/pnmimage_base.cxx
index 1ff9f3a710..fd64f59982 100644
--- a/panda/src/pnmimage/pnmimage_base.cxx
+++ b/panda/src/pnmimage/pnmimage_base.cxx
@@ -33,7 +33,13 @@ pm_message(const char *format, ...) {
 
   static const size_t buffer_size = 1024;
   char buffer[buffer_size];
+#ifdef WIN32_VC
+  // Windows doesn't define vsnprintf().  Hope we don't overflow.
+  vsprintf(buffer, format, ap);
+#else
   vsnprintf(buffer, buffer_size, format, ap);
+#endif
+  nassertv(strlen(buffer) < buffer_size);
 
   pnmimage_cat.info() << buffer << "\n";
 
@@ -53,7 +59,13 @@ pm_error(const char *format, ...) {
 
   static const size_t buffer_size = 1024;
   char buffer[buffer_size];
+#ifdef WIN32_VC
+  // Windows doesn't define vsnprintf().  Hope we don't overflow.
+  vsprintf(buffer, format, ap);
+#else
   vsnprintf(buffer, buffer_size, format, ap);
+#endif
+  nassertv(strlen(buffer) < buffer_size);
 
   pnmimage_cat.error() << buffer << "\n";