diff --git a/diff.c b/diff.c
index 5df28e49c5..1771b2c444 100644
--- a/diff.c
+++ b/diff.c
@@ -60,7 +60,7 @@ static int diff_suppress_blank_empty;
 static enum git_colorbool diff_use_color_default = GIT_COLOR_UNKNOWN;
 static int diff_color_moved_default;
 static int diff_color_moved_ws_default;
-static int diff_context_default = 3;
+static unsigned int diff_context_default = 3;
 static unsigned int diff_interhunk_context_default;
 static char *diff_word_regex_cfg;
 static struct external_diff external_diff_cfg;
@@ -382,9 +382,10 @@ int git_diff_ui_config(const char *var, const char *value,
 		return 0;
 	}
 	if (!strcmp(var, "diff.context")) {
-		diff_context_default = git_config_int(var, value, ctx->kvi);
-		if (diff_context_default < 0)
+		int val = git_config_int(var, value, ctx->kvi);
+		if (val < 0)
 			return -1;
+		diff_context_default = val;
 		return 0;
 	}
 	if (!strcmp(var, "diff.interhunkcontext")) {
@@ -5924,9 +5925,12 @@ static int diff_opt_unified(const struct option *opt,
 	BUG_ON_OPT_NEG(unset);
 
 	if (arg) {
-		options->context = strtol(arg, &s, 10);
+		long val = strtol(arg, &s, 10);
 		if (*s)
 			return error(_("%s expects a numerical value"), "--unified");
+		if (val < 0)
+			return error(_("%s expects a non-negative integer"), "--unified");
+		options->context = val;
 	}
 	enable_patch_output(&options->output_format);
 
diff --git a/diff.h b/diff.h
index 033d633db4..bb5cddaf34 100644
--- a/diff.h
+++ b/diff.h
@@ -294,7 +294,7 @@ struct diff_options {
 	enum git_colorbool use_color;
 
 	/* Number of context lines to generate in patch output. */
-	int context;
+	unsigned int context;
 
 	unsigned int interhunkcontext;
 
diff --git a/t/t4055-diff-context.sh b/t/t4055-diff-context.sh
index 1384a81957..b26f6eea7c 100755
--- a/t/t4055-diff-context.sh
+++ b/t/t4055-diff-context.sh
@@ -82,6 +82,11 @@ test_expect_success 'negative integer config parsing' '
 	test_grep "bad config variable" output
 '
 
+test_expect_success '-U-1 is rejected' '
+	test_must_fail git diff -U-1 2>err &&
+	test_grep "expects a non-negative integer" err
+'
+
 test_expect_success '-U0 is valid, so is diff.context=0' '
 	test_config diff.context 0 &&
 	git diff >output &&