diff --git a/runner/filteroperator.go b/runner/filteroperator.go
index 69f93cca..8e896249 100644
--- a/runner/filteroperator.go
+++ b/runner/filteroperator.go
@@ -34,7 +34,14 @@ func (f FilterOperator) Parse(flagValue string) (string, time.Duration, error) {
 			timeVal := strings.Trim(splittedFlagValue[1], " ")
 			value, err = time.ParseDuration(timeVal)
 			if err != nil && strings.Contains(err.Error(), "missing unit") {
-				value, _ = time.ParseDuration(fmt.Sprintf("%ss", timeVal))
+				// A bare number is read as seconds. This retry used to drop its
+				// error, so a number too large to hold as a duration left value
+				// at zero and reported nothing, and the filter then matched
+				// every host instead of rejecting the flag.
+				value, err = time.ParseDuration(fmt.Sprintf("%ss", timeVal))
+				if err != nil {
+					return operator, value, fmt.Errorf("invalid value provided for %s", f.flag)
+				}
 			} else if err != nil {
 				return operator, value, fmt.Errorf("invalid value provided for %s", f.flag)
 			}
diff --git a/runner/filteroperator_test.go b/runner/filteroperator_test.go
new file mode 100644
index 00000000..ced8a4b6
--- /dev/null
+++ b/runner/filteroperator_test.go
@@ -0,0 +1,62 @@
+package runner
+
+import (
+	"testing"
+	"time"
+
+	"github.com/stretchr/testify/require"
+)
+
+func TestFilterOperatorParse(t *testing.T) {
+	f := FilterOperator{flag: "-mrt, -match-response-time"}
+
+	t.Run("valid", func(t *testing.T) {
+		for _, tc := range []struct {
+			in       string
+			operator string
+			value    time.Duration
+		}{
+			{in: "<1s", operator: "<", value: time.Second},
+			{in: ">1s", operator: ">", value: time.Second},
+			{in: "=1s", operator: "=", value: time.Second},
+			{in: "<=1s", operator: "<=", value: time.Second},
+			{in: ">=1s", operator: ">=", value: time.Second},
+			{in: "!=1s", operator: "!=", value: time.Second},
+			{in: "<100ms", operator: "<", value: 100 * time.Millisecond},
+			// A bare number is read as seconds.
+			{in: ">=5", operator: ">=", value: 5 * time.Second},
+			// Surrounding spaces are trimmed.
+			{in: ">= 5s ", operator: ">=", value: 5 * time.Second},
+		} {
+			t.Run(tc.in, func(t *testing.T) {
+				operator, value, err := f.Parse(tc.in)
+				require.NoError(t, err)
+				require.Equal(t, tc.operator, operator)
+				require.Equal(t, tc.value, value)
+			})
+		}
+	})
+
+	t.Run("rejected", func(t *testing.T) {
+		for _, in := range []string{
+			// No operator at all.
+			"1s",
+			"",
+			// An operator with nothing after it.
+			">=",
+			// Not a number and not a duration.
+			">=abc",
+			// A bare number too large to hold as a duration once seconds are
+			// added. This used to come back as 0 with no error, which turned
+			// -mrt into a filter that matched every host.
+			">=10000000000",
+			">=99999999999999999999",
+		} {
+			t.Run(in, func(t *testing.T) {
+				_, value, err := f.Parse(in)
+				require.Error(t, err, "expected %q to be rejected", in)
+				require.Zero(t, value)
+			})
+		}
+	})
+}