diff --git a/misc.c b/misc.c
index 7f8d6df..64da6de 100644
--- a/misc.c
+++ b/misc.c
@@ -54,7 +54,7 @@ wait_for_vcp(void)
 
 #if MEM_DEBUG_BUF
 static char debugbuf[1024];
-static uint32_t debugpos;
+static uint32_t debugpos = 0;
 
 void
 debug_printf(const char *format, ...)
@@ -75,6 +75,9 @@ debug_printf(const char *format, ...)
 		memcpy(&debugbuf[debugpos], pos, cpy);
 
 		debugpos += cpy;
+		if (debugpos >= sizeof debugbuf)
+			debugpos = 0;
+
 		pos += cpy;
 		length -= cpy;
 	}