--- sys/kern/subr_sbuf.c.orig
+++ sys/kern/subr_sbuf.c
@@ -333,8 +333,15 @@
 		s->s_error = -len;
 		return (s->s_error);
 	}
-	KASSERT(len > 0 && len <= s->s_len,
-	    ("Bad drain amount %d for sbuf %p", len, s));
+	if (len == 0 || len > s->s_len) {
+		/*
+		 * Drain callback violated its contract.  Stop before it
+		 * corrupts the buffer: the KASSERT alone is compiled out
+		 * of production (non-INVARIANTS) kernels.
+		 */
+		s->s_error = EDOOFUS;
+		return (s->s_error);
+	}
 	s->s_len -= len;
 	/*
 	 * Fast path for the expected case where all the data was