diff --git a/polymer/eduke32/build/include/compat.h b/polymer/eduke32/build/include/compat.h
index add4e02fa..d03b0cc76 100644
--- a/polymer/eduke32/build/include/compat.h
+++ b/polymer/eduke32/build/include/compat.h
@@ -675,6 +675,7 @@ char *Bstrlwr(char *);
 char *Bstrupr(char *);
 #endif
 
+// Copy min(strlen(src)+1, n) characters into dst, always terminate with a NUL.
 static inline char *Bstrncpyz(char *dst, const char *src, bsize_t n)
 {
     Bstrncpy(dst, src, n);
@@ -682,6 +683,19 @@ static inline char *Bstrncpyz(char *dst, const char *src, bsize_t n)
     return dst;
 }
 
+// Append extension when <outbuf> contains no dot.
+// <ext> should be like ".mhk", i.e. it MUST start with a dot.
+// The ugly name is deliberate: we should be checking the sizes of all buffers!
+static inline void append_ext_UNSAFE(char *outbuf, const char *ext)
+{
+    char *p = Bstrrchr(outbuf,'.');
+
+    if (!p)
+        Bstrcat(outbuf, ext);
+    else
+        Bstrcpy(p+1, ext+1);
+}
+
 #ifdef EXTERNC
 }
 #endif
diff --git a/polymer/eduke32/build/src/build.c b/polymer/eduke32/build/src/build.c
index b9e212bfe..feacc55f9 100644
--- a/polymer/eduke32/build/src/build.c
+++ b/polymer/eduke32/build/src/build.c
@@ -822,22 +822,13 @@ CANCEL:
 static int32_t mhk=0;
 static void loadmhk(int32_t domessage)
 {
-    char *p; char levname[BMAX_PATH];
+    char levname[BMAX_PATH];
 
     if (!mhk)
         return;
 
     Bstrcpy(levname, boardfilename);
-    p = Bstrrchr(levname,'.');
-    if (!p)
-        Bstrcat(levname,".mhk");
-    else
-    {
-        p[1]='m';
-        p[2]='h';
-        p[3]='k';
-        p[4]=0;
-    }
+    append_ext_UNSAFE(levname, ".mhk");
 
     if (!loadmaphack(levname))
     {
diff --git a/polymer/eduke32/build/src/engine.c b/polymer/eduke32/build/src/engine.c
index c609a968e..79b7235d6 100644
--- a/polymer/eduke32/build/src/engine.c
+++ b/polymer/eduke32/build/src/engine.c
@@ -9932,21 +9932,10 @@ int32_t loadboard(char *filename, char flags, vec3_t *dapos, int16_t *daang, int
 #endif
 
     {
-        char *p, fn[BMAX_PATH];
+        char fn[BMAX_PATH];
 
         Bstrcpy(fn, filename);
-
-        p = Bstrrchr(fn, '.');
-
-        if (!p)
-            Bstrcat(fn, ".cfg");
-        else
-        {
-            p[1]='c';
-            p[2]='f';
-            p[3]='g';
-            p[4]=0;
-        }
+        append_ext_UNSAFE(fn, ".cfg");
 
         OSD_Exec(fn);
     }
diff --git a/polymer/eduke32/source/premap.c b/polymer/eduke32/source/premap.c
index b270133cf..cbd0df2da 100644
--- a/polymer/eduke32/source/premap.c
+++ b/polymer/eduke32/source/premap.c
@@ -1759,16 +1759,7 @@ static void G_LoadMapHack(char *outbuf, const char *filename)
     if (filename != NULL)
         Bstrcpy(outbuf, filename);
 
-    p = Bstrrchr(outbuf,'.');
-    if (!p)
-        Bstrcat(outbuf,".mhk");
-    else
-    {
-        p[1]='m';
-        p[2]='h';
-        p[3]='k';
-        p[4]=0;
-    }
+    append_ext_UNSAFE(outbuf, ".mhk");
 
     if (!loadmaphack(outbuf))
         initprintf("Loaded map hack file \"%s\"\n",outbuf);
diff --git a/polymer/eduke32/source/savegame.c b/polymer/eduke32/source/savegame.c
index 23bac7aa6..a274e63e3 100644
--- a/polymer/eduke32/source/savegame.c
+++ b/polymer/eduke32/source/savegame.c
@@ -285,18 +285,7 @@ int32_t G_LoadPlayer(int32_t spot)
     // TODO: this stuff needs to be factored out, too...
     if (currentboardfilename[0])
     {
-        char *p;
-
-        p = Bstrrchr(currentboardfilename,'.');
-        if (!p) Bstrcat(currentboardfilename,".mhk");
-        else
-        {
-            p[1]='m';
-            p[2]='h';
-            p[3]='k';
-            p[4]=0;
-        }
-
+        append_ext_UNSAFE(currentboardfilename, ".mhk");
         loadmaphack(currentboardfilename);
     }