diff --git a/internal/appconfig/serde.go b/internal/appconfig/serde.go
index f429986300..be9a473cc3 100644
--- a/internal/appconfig/serde.go
+++ b/internal/appconfig/serde.go
@@ -290,7 +290,7 @@ func stringifyYAMLMapKeys(obj any) any {
 	} else if mapobj, ok := obj.(map[any]any); ok {
 		newmap := make(map[string]any)
 		for k, v := range mapobj {
-			newmap[k.(string)] = stringifyYAMLMapKeys(v)
+			newmap[fmt.Sprint(k)] = stringifyYAMLMapKeys(v)
 		}
 		obj = newmap
 	}
diff --git a/internal/appconfig/serde_test.go b/internal/appconfig/serde_test.go
index 8645e72ef4..47f61990d0 100644
--- a/internal/appconfig/serde_test.go
+++ b/internal/appconfig/serde_test.go
@@ -780,6 +780,25 @@ func TestYAMLPrettyPrint(t *testing.T) {
 	assert.Contains(t, string(buf), "\n    processes:\n      - web\n")
 }
 
+func TestLoadYAMLAppConfigWithNonStringKeys(t *testing.T) {
+	dir := t.TempDir()
+	path := filepath.Join(dir, "fly.yaml")
+	err := os.WriteFile(path, []byte("app: foo\nenv:\n  1: one\n  true: yes\n"), 0o600)
+	require.NoError(t, err)
+
+	require.NotPanics(t, func() {
+		cfg, err := LoadConfig(path)
+		assert.NoError(t, err)
+		assert.Equal(t, "foo", cfg.AppName)
+		assert.Equal(t, "one", cfg.Env["1"])
+		assert.Equal(t, "yes", cfg.Env["true"])
+
+		cfgMap, err := LoadConfigAsMap(path)
+		assert.NoError(t, err)
+		assert.Equal(t, "foo", cfgMap["app"])
+	})
+}
+
 //go:fix inline
 func UintPointer(v uint32) *uint32 {
 	return new(v)