diff --git a/dlls/kernel32/debugger.c b/dlls/kernel32/debugger.c
index 7e1a860adc8..528ba763892 100644
--- a/dlls/kernel32/debugger.c
+++ b/dlls/kernel32/debugger.c
@@ -245,6 +245,8 @@ void WINAPI OutputDebugStringA( LPCSTR str )
     static HANDLE DBWinMutex = NULL;
     static BOOL mutex_inited = FALSE;
 
+    if (!str) str = "";
+
     /* send string to attached debugger */
     SERVER_START_REQ( output_debug_string )
     {