diff --git a/internal/cli/run.go b/internal/cli/run.go
index d141484..1fe9422 100644
--- a/internal/cli/run.go
+++ b/internal/cli/run.go
@@ -547,7 +547,7 @@ func parseFlags(args []string, stderr io.Writer) (options, error) {
 		return options{}, errors.New("--max-retries must be 0 or greater")
 	}
 	if opts.ChunkSize <= 0 {
-		opts.ChunkSize = defaultChunkSize
+		return options{}, errors.New("--chunk-size must be positive")
 	}
 
 	opts.SourceURLs = fs.Args()
diff --git a/internal/cli/run_test.go b/internal/cli/run_test.go
index c505328..e7022da 100644
--- a/internal/cli/run_test.go
+++ b/internal/cli/run_test.go
@@ -520,6 +520,16 @@ func TestParseFlagsRejectsInvalidMaxRetries(t *testing.T) {
 	}
 }
 
+func TestParseFlagsRejectsNonPositiveChunkSize(t *testing.T) {
+	testCases := []string{"0", "-1"}
+	for _, value := range testCases {
+		_, err := parseFlags([]string{"--chunk-size", value, "https://example.com"}, io.Discard)
+		if err == nil || !strings.Contains(err.Error(), "--chunk-size must be positive") {
+			t.Fatalf("parseFlags error=%v, want chunk-size validation error for value=%s", err, value)
+		}
+	}
+}
+
 func TestParseFlagsAllowsVersionWithoutURL(t *testing.T) {
 	opts, err := parseFlags([]string{"--version"}, io.Discard)
 	if err != nil {