diff --git a/src/ast/nodes/shared/FunctionBase.ts b/src/ast/nodes/shared/FunctionBase.ts
index 76b48c7bb4f..e6fc3fb83b7 100644
--- a/src/ast/nodes/shared/FunctionBase.ts
+++ b/src/ast/nodes/shared/FunctionBase.ts
@@ -79,8 +79,8 @@ export default abstract class FunctionBase extends NodeBase {
 	private deoptimizeParameterVariableValues() {
 		for (const parameter of this.params) {
 			if (parameter instanceof Identifier) {
-				const parameterVariable = parameter.variable as ParameterVariable | null;
-				parameterVariable?.markReassigned();
+				const parameterVariable = parameter.variable as ParameterVariable;
+				parameterVariable.markReassigned();
 			}
 		}
 	}
diff --git a/src/ast/variables/ParameterVariable.ts b/src/ast/variables/ParameterVariable.ts
index 2115048dc9a..a17bf3b1152 100644
--- a/src/ast/variables/ParameterVariable.ts
+++ b/src/ast/variables/ParameterVariable.ts
@@ -143,7 +143,7 @@ export default class ParameterVariable extends LocalVariable {
 	 * The optimization can be undone by calling `markReassigned`.
 	 * @returns the frozen value
 	 */
-	getKnownValue(): ExpressionEntity {
+	private getKnownValue(): ExpressionEntity {
 		if (this.frozenValue === null) {
 			this.frozenValue = this.knownValue || UNKNOWN_EXPRESSION;
 		}