diff --git a/.prettierignore b/.prettierignore
new file mode 100644
index 0000000..6461dee
--- /dev/null
+++ b/.prettierignore
@@ -0,0 +1 @@
+*.ts
diff --git a/src/internal/args.ts b/src/internal/args.ts
index 3de3190..9181829 100644
--- a/src/internal/args.ts
+++ b/src/internal/args.ts
@@ -116,6 +116,9 @@ export interface WithFallbackConfig extends
 export const isArgs = (u: unknown): u is Args.Args<unknown> =>
   typeof u === "object" && u != null && ArgsTypeId in u
 
+/** @internal */
+export const isInstruction = <_>(self: Args.Args<_>): self is Instruction => self as any
+
 /** @internal */
 export const isEmpty = (self: Instruction): self is Empty => self._tag === "Empty"
 
@@ -131,6 +134,13 @@ export const isMap = (self: Instruction): self is Map => self._tag === "Map"
 /** @internal */
 export const isVariadic = (self: Instruction): self is Variadic => self._tag === "Variadic"
 
+/** @internal */
+export const isWithDefault = (self: Instruction): self is WithDefault => self._tag === "WithDefault"
+
+/** @internal */
+export const isWithFallbackConfig = (self: Instruction): self is WithFallbackConfig =>
+  self._tag === "WithFallbackConfig"
+
 // =============================================================================
 // Constructors
 // =============================================================================
@@ -351,11 +361,10 @@ export const withFallbackConfig: {
   <B>(config: Config.Config<B>) => <A>(self: Args.Args<A>) => Args.Args<A | B>,
   <A, B>(self: Args.Args<A>, config: Config.Config<B>) => Args.Args<A | B>
 >(2, (self, config) => {
-  if ((self as Instruction)._tag === "WithDefault") {
-    const withDefault = self as WithDefault
+  if (isInstruction(self) && isWithDefault(self)) {
     return makeWithDefault(
-      withFallbackConfig(withDefault.args, config),
-      withDefault.fallback as any
+      withFallbackConfig(self.args, config),
+      self.fallback as any
     )
   }
   return makeWithFallbackConfig(self, config)
diff --git a/src/internal/options.ts b/src/internal/options.ts
index c060a53..53b4ca8 100644
--- a/src/internal/options.ts
+++ b/src/internal/options.ts
@@ -144,6 +144,9 @@ export interface WithDefault extends
 export const isOptions = (u: unknown): u is Options.Options<unknown> =>
   typeof u === "object" && u != null && OptionsTypeId in u
 
+/** @internal */
+export const isInstruction = <_>(self: Options.Options<_>): self is Instruction => self as any
+
 /** @internal */
 export const isEmpty = (self: Instruction): self is Empty => self._tag === "Empty"
 
@@ -165,6 +168,10 @@ export const isOrElse = (self: Instruction): self is OrElse => self._tag === "Or
 /** @internal */
 export const isWithDefault = (self: Instruction): self is WithDefault => self._tag === "WithDefault"
 
+/** @internal */
+export const isWithFallbackConfig = (self: Instruction): self is WithFallbackConfig =>
+  self._tag === "WithFallbackConfig"
+
 // =============================================================================
 // Constructors
 // =============================================================================
@@ -540,11 +547,10 @@ export const withFallbackConfig: {
   <B>(config: Config.Config<B>) => <A>(self: Options.Options<A>) => Options.Options<A | B>,
   <A, B>(self: Options.Options<A>, config: Config.Config<B>) => Options.Options<A | B>
 >(2, (self, config) => {
-  if ((self as Instruction)._tag === "WithDefault") {
-    const withDefault = self as WithDefault
+  if (isInstruction(self) && isWithDefault(self)) {
     return makeWithDefault(
-      withFallbackConfig(withDefault.options, config),
-      withDefault.fallback as any
+      withFallbackConfig(self.options, config),
+      self.fallback as any
     )
   }
   return makeWithFallbackConfig(self, config)