diff --git a/.changeset/fix-not-regex-value-types.md b/.changeset/fix-not-regex-value-types.md
new file mode 100644
index 0000000..6757a54
--- /dev/null
+++ b/.changeset/fix-not-regex-value-types.md
@@ -0,0 +1,5 @@
+---
+"posthog-go": patch
+---
+
+Fix `not_regex` local flag evaluation erroring on non-string/int property values. The `not_regex` operator used a manual string/int type switch and returned an error for other types, most notably `float64`, which is what JSON numbers deserialize to, so it failed on a numeric property value even though `regex` handled it. `not_regex` now coerces both sides with `valueToString`, mirroring `regex`. An explicit `nil` property value is represented as `null`, matching the feature flags evaluation service rather than Go's default `<nil>` representation.
diff --git a/feature_flags_matching_test.go b/feature_flags_matching_test.go
index b74892d..90a7d6f 100644
--- a/feature_flags_matching_test.go
+++ b/feature_flags_matching_test.go
@@ -309,6 +309,34 @@ func TestMatchPropertyRegex(t *testing.T) {
 	}
 }
 
+func TestMatchPropertyNotRegexHandlesAllValueTypes(t *testing.T) {
+	// not_regex must coerce like regex does. JSON numbers arrive as float64,
+	// which the old manual type switch rejected with an error even though
+	// regex handled it.
+	for _, ov := range []interface{}{"123", 123, 123.0, float64(123)} {
+		m, err := matchProperty(FlagProperty{Key: "k", Value: "^1", Operator: "not_regex"}, NewProperties().Set("k", ov))
+		require.NoError(t, err)
+		require.False(t, m) // "123" matches ^1, so not_regex is false
+	}
+	m, err := matchProperty(FlagProperty{Key: "k", Value: "^9", Operator: "not_regex"}, NewProperties().Set("k", 123.0))
+	require.NoError(t, err)
+	require.True(t, m) // "123" does not match ^9
+
+	// The feature flags evaluation service stringifies a null property as "null"
+	// before matching it. not_regex must negate the regex result as usual.
+	m, err = matchProperty(FlagProperty{Key: "k", Value: "^1", Operator: "not_regex"}, NewProperties().Set("k", nil))
+	require.NoError(t, err)
+	require.True(t, m)
+	m, err = matchProperty(FlagProperty{Key: "k", Value: "^null$", Operator: "not_regex"}, NewProperties().Set("k", nil))
+	require.NoError(t, err)
+	require.False(t, m)
+	// Regression guard: Go's default "<nil>" representation must not leak into
+	// local evaluation.
+	m, err = matchProperty(FlagProperty{Key: "k", Value: "^<nil>$", Operator: "not_regex"}, NewProperties().Set("k", nil))
+	require.NoError(t, err)
+	require.True(t, m)
+}
+
 func TestMatchPropertyContains(t *testing.T) {
 	shouldMatch := []interface{}{"value", "value2", "value3", "value4", "343tfvalue5"}
 
diff --git a/featureflags.go b/featureflags.go
index 6696399..66dfaaa 100644
--- a/featureflags.go
+++ b/featureflags.go
@@ -1215,32 +1215,24 @@ func matchProperty(property FlagProperty, properties Properties) (bool, error) {
 	}
 
 	if operator == "not_regex" {
-		var pattern string
-
-		if valueString, ok := value.(string); ok {
-			pattern = valueString
-		} else if valueInt, ok := value.(int); ok {
-			pattern = strconv.Itoa(valueInt)
-		} else {
-			return false, errors.New("regex expression not allowed")
-		}
-
-		r, err := getOrCompileRegex(pattern)
+		// The feature flags evaluation service stringifies an explicit JSON null
+		// as "null" before regex matching. Avoid Go's default "<nil>" representation.
+		overrideValueString := valueToString(override_value)
+		if override_value == nil {
+			overrideValueString = "null"
+		}
+		// Mirror the "regex" branch above: coerce both sides with valueToString
+		// so all property/flag value types are handled. The previous manual
+		// string/int type switch errored on other types, most notably float64,
+		// which is what JSON numbers deserialize to, so not_regex failed on a
+		// numeric property value even though regex handled it.
+		r, err := getOrCompileRegex(valueToString(value))
 		// invalid regex
 		if err != nil {
 			return false, nil
 		}
 
-		var match bool
-		if valueString, ok := override_value.(string); ok {
-			match = r.MatchString(valueString)
-		} else if valueInt, ok := override_value.(int); ok {
-			match = r.MatchString(strconv.Itoa(valueInt))
-		} else {
-			return false, errors.New("value type not supported")
-		}
-
-		return !match, nil
+		return !r.MatchString(overrideValueString), nil
 	}
 
 	if operator == "gt" {