diff --git a/packages/compiler-cli/ngcc/src/host/delegating_host.ts b/packages/compiler-cli/ngcc/src/host/delegating_host.ts
index d1e7293feb..b7cc897e84 100644
--- a/packages/compiler-cli/ngcc/src/host/delegating_host.ts
+++ b/packages/compiler-cli/ngcc/src/host/delegating_host.ts
@@ -32,7 +32,8 @@ export class DelegatingReflectionHost implements NgccReflectionHost {
 
   getDeclarationOfIdentifier(id: ts.Identifier): Declaration|null {
     if (isFromDtsFile(id)) {
-      return this.detectKnownDeclaration(this.tsHost.getDeclarationOfIdentifier(id));
+      const declaration = this.tsHost.getDeclarationOfIdentifier(id);
+      return declaration !== null ? this.detectKnownDeclaration(declaration) : null;
     }
     return this.ngccHost.getDeclarationOfIdentifier(id);
   }
@@ -157,10 +158,7 @@ export class DelegatingReflectionHost implements NgccReflectionHost {
     return this.ngccHost.getEndOfClass(classSymbol);
   }
 
-  detectKnownDeclaration(decl: null): null;
-  detectKnownDeclaration<T extends Declaration>(decl: T): T;
-  detectKnownDeclaration<T extends Declaration>(decl: T|null): T|null;
-  detectKnownDeclaration<T extends Declaration>(decl: T|null): T|null {
+  detectKnownDeclaration<T extends Declaration>(decl: T): T {
     return this.ngccHost.detectKnownDeclaration(decl);
   }
 }
diff --git a/packages/compiler-cli/ngcc/src/host/esm2015_host.ts b/packages/compiler-cli/ngcc/src/host/esm2015_host.ts
index a401f6ea64..31b88bded5 100644
--- a/packages/compiler-cli/ngcc/src/host/esm2015_host.ts
+++ b/packages/compiler-cli/ngcc/src/host/esm2015_host.ts
@@ -604,16 +604,12 @@ export class Esm2015ReflectionHost extends TypeScriptReflectionHost implements N
    * @param decl The `Declaration` to check.
    * @return The passed in `Declaration` (potentially enhanced with a `KnownDeclaration`).
    */
-  detectKnownDeclaration(decl: null): null;
-  detectKnownDeclaration<T extends Declaration>(decl: T): T;
-  detectKnownDeclaration<T extends Declaration>(decl: T|null): T|null;
-  detectKnownDeclaration<T extends Declaration>(decl: T|null): T|null {
-    if (decl !== null && decl.known === null && this.isJavaScriptObjectDeclaration(decl)) {
+  detectKnownDeclaration<T extends Declaration>(decl: T): T {
+    if (decl.known === null && this.isJavaScriptObjectDeclaration(decl)) {
       // If the identifier resolves to the global JavaScript `Object`, update the declaration to
       // denote it as the known `JsGlobalObject` declaration.
       decl.known = KnownDeclaration.JsGlobalObject;
     }
-
     return decl;
   }
 
@@ -626,7 +622,11 @@ export class Esm2015ReflectionHost extends TypeScriptReflectionHost implements N
    */
   protected getDeclarationOfSymbol(symbol: ts.Symbol, originalId: ts.Identifier|null): Declaration
       |null {
-    return this.detectKnownDeclaration(super.getDeclarationOfSymbol(symbol, originalId));
+    const declaration = super.getDeclarationOfSymbol(symbol, originalId);
+    if (declaration === null) {
+      return null;
+  }
+    return this.detectKnownDeclaration(declaration);
   }
 
   /**
diff --git a/packages/compiler-cli/ngcc/src/host/esm5_host.ts b/packages/compiler-cli/ngcc/src/host/esm5_host.ts
index f620e38243..b93d37d968 100644
--- a/packages/compiler-cli/ngcc/src/host/esm5_host.ts
+++ b/packages/compiler-cli/ngcc/src/host/esm5_host.ts
@@ -283,13 +283,11 @@ export class Esm5ReflectionHost extends Esm2015ReflectionHost {
    * @param decl The `Declaration` to check.
    * @return The passed in `Declaration` (potentially enhanced with a `KnownDeclaration`).
    */
-  detectKnownDeclaration(decl: null): null;
-  detectKnownDeclaration<T extends Declaration>(decl: T): T;
-  detectKnownDeclaration<T extends Declaration>(decl: T|null): T|null;
-  detectKnownDeclaration<T extends Declaration>(decl: T|null): T|null {
+  detectKnownDeclaration<T extends Declaration>(decl: T): T {
     decl = super.detectKnownDeclaration(decl);
 
-    if (decl !== null && decl.known === null && decl.node !== null) {
+    // Also check for TS helpers
+    if (decl.known === null && decl.node !== null) {
       decl.known = getTsHelperFnFromDeclaration(decl.node);
     }
 
diff --git a/packages/compiler-cli/ngcc/src/host/ngcc_host.ts b/packages/compiler-cli/ngcc/src/host/ngcc_host.ts
index 2d74d0532b..a6165e7bb4 100644
--- a/packages/compiler-cli/ngcc/src/host/ngcc_host.ts
+++ b/packages/compiler-cli/ngcc/src/host/ngcc_host.ts
@@ -100,8 +100,8 @@ export interface NgccReflectionHost extends ReflectionHost {
    * Check whether a `Declaration` corresponds with a known declaration and set its `known` property
    * to the appropriate `KnownDeclaration`.
    *
-   * @param decl The `Declaration` to check or `null` if there is no declaration.
+   * @param decl The `Declaration` to check.
    * @return The passed in `Declaration` (potentially enhanced with a `KnownDeclaration`).
    */
-  detectKnownDeclaration<T extends Declaration>(decl: T|null): T|null;
+  detectKnownDeclaration<T extends Declaration>(decl: T): T;
 }