diff --git a/src/utils/ensureArray.ts b/src/utils/ensureArray.ts
index 080d73ae226..0f88c70b20a 100644
--- a/src/utils/ensureArray.ts
+++ b/src/utils/ensureArray.ts
@@ -1,8 +1,6 @@
-export function ensureArray<T>(
-	items: (T | false | null | undefined)[] | T | false | null | undefined
-): T[] {
+export function ensureArray<T>(items: T | T[]): T[] {
 	if (Array.isArray(items)) {
-		return items.filter(Boolean) as T[];
+		return items.filter(Boolean);
 	}
 	if (items) {
 		return [items];
diff --git a/src/utils/options/mergeOptions.ts b/src/utils/options/mergeOptions.ts
index 9b92961d4b4..bc73f3df399 100644
--- a/src/utils/options/mergeOptions.ts
+++ b/src/utils/options/mergeOptions.ts
@@ -156,7 +156,7 @@ const getExternal = (
 	return typeof configExternal === 'function'
 		? (source: string, importer: string | undefined, isResolved: boolean) =>
 				configExternal(source, importer, isResolved) || overrides.external.indexOf(source) !== -1
-		: ensureArray(configExternal).concat(overrides.external);
+		: (ensureArray(configExternal).concat(overrides.external) as ExternalOption);
 };
 
 const getOnWarn = (
diff --git a/src/utils/options/normalizeInputOptions.ts b/src/utils/options/normalizeInputOptions.ts
index de12b6dd33e..a086d680705 100644
--- a/src/utils/options/normalizeInputOptions.ts
+++ b/src/utils/options/normalizeInputOptions.ts
@@ -4,6 +4,7 @@ import {
 	InputOptions,
 	ModuleSideEffectsOption,
 	NormalizedInputOptions,
+	Plugin,
 	PreserveEntrySignaturesOption,
 	PureModulesOption,
 	RollupBuild,
@@ -53,7 +54,7 @@ export function normalizeInputOptions(config: InputOptions): {
 		moduleContext: getModuleContext(config, context),
 		onwarn,
 		perf: config.perf || false,
-		plugins: ensureArray(config.plugins),
+		plugins: ensureArray(config.plugins) as Plugin[],
 		preserveEntrySignatures: getPreserveEntrySignatures(config, unsetOptions),
 		preserveModules: getPreserveModules(config, onwarn, strictDeprecations),
 		preserveSymlinks: config.preserveSymlinks || false,
@@ -101,7 +102,8 @@ const getAcorn = (config: InputOptions): acorn.Options => ({
 
 const getAcornInjectPlugins = (
 	config: InputOptions
-): NormalizedInputOptions['acornInjectPlugins'] => ensureArray(config.acornInjectPlugins);
+): NormalizedInputOptions['acornInjectPlugins'] =>
+	ensureArray(config.acornInjectPlugins) as (() => unknown)[];
 
 const getCache = (config: InputOptions): NormalizedInputOptions['cache'] =>
 	(config.cache as unknown as RollupBuild)?.cache || config.cache;