diff --git a/include/assert.h b/include/assert.h
index a4dcd5d7f..ca8d4acc1 100644
--- a/include/assert.h
+++ b/include/assert.h
@@ -42,8 +42,8 @@
 #ifdef NDEBUG
 #  define assert(expr)
 #else
-extern void __fastcall__ _afailed (const char*, unsigned);
-#  define assert(expr)  ((expr)? (void)0 : _afailed(__FILE__, __LINE__))
+extern void __fastcall__ __afailed (const char*, unsigned);
+#  define assert(expr)  ((expr)? (void)0 : __afailed(__FILE__, __LINE__))
 #endif
 
 /* TODO: Guard with #if __CC65_STD__ >= __CC65_STD_C11__ if there
diff --git a/libsrc/common/_afailed.c b/libsrc/common/_afailed.c
index ab8b94ca6..01e8d5397 100644
--- a/libsrc/common/_afailed.c
+++ b/libsrc/common/_afailed.c
@@ -12,7 +12,7 @@
 #include <stdlib.h>
 
 
-void __fastcall__ _afailed (char* file, unsigned line)
+void __fastcall__ __afailed (char* file, unsigned line)
 {
     raise (SIGABRT);
     fprintf (stderr, "ASSERTION FAILED IN %s:%u\n", file, line);
diff --git a/libsrc/geos-common/common/_afailed.c b/libsrc/geos-common/common/_afailed.c
index 97727d605..acc437fb0 100644
--- a/libsrc/geos-common/common/_afailed.c
+++ b/libsrc/geos-common/common/_afailed.c
@@ -8,7 +8,7 @@
 #include <stdlib.h>
 #include <geos.h>
 
-void _afailed (char* file, unsigned line)
+void __afailed (char* file, unsigned line)
 {
     ExitTurbo();