diff --git a/builtin-grep.c b/builtin-grep.c
index 185876b..bbf747f 100644
--- a/builtin-grep.c
+++ b/builtin-grep.c
@@ -294,7 +294,7 @@
 			if (opt->pre_context) {
 				push_arg("-B");
 				len += snprintf(argptr, sizeof(randarg)-len,
-						"%u", opt->pre_context);
+						"%u", opt->pre_context) + 1;
 				if (sizeof(randarg) <= len)
 					die("maximum length of args exceeded");
 				push_arg(argptr);
@@ -303,7 +303,7 @@
 			if (opt->post_context) {
 				push_arg("-A");
 				len += snprintf(argptr, sizeof(randarg)-len,
-						"%u", opt->post_context);
+						"%u", opt->post_context) + 1;
 				if (sizeof(randarg) <= len)
 					die("maximum length of args exceeded");
 				push_arg(argptr);
@@ -313,7 +313,7 @@
 		else {
 			push_arg("-C");
 			len += snprintf(argptr, sizeof(randarg)-len,
-					"%u", opt->post_context);
+					"%u", opt->post_context) + 1;
 			if (sizeof(randarg) <= len)
 				die("maximum length of args exceeded");
 			push_arg(argptr);