diff --git a/src/compiler/checker.ts b/src/compiler/checker.ts
index 38d50f9d19d..21e7ae28da9 100644
--- a/src/compiler/checker.ts
+++ b/src/compiler/checker.ts
@@ -7200,15 +7200,15 @@ namespace ts {
             return symbol.members.get(InternalSymbolName.Index);
         }
 
-        function getIndexDeclarationOfSymbol(symbol: Symbol, kind: IndexKind): SignatureDeclaration {
+        function getIndexDeclarationOfSymbol(symbol: Symbol, kind: IndexKind): IndexSignatureDeclaration {
             const syntaxKind = kind === IndexKind.Number ? SyntaxKind.NumberKeyword : SyntaxKind.StringKeyword;
             const indexSymbol = getIndexSymbol(symbol);
             if (indexSymbol) {
                 for (const decl of indexSymbol.declarations) {
-                    const node = <SignatureDeclaration>decl;
+                    const node = cast(decl, isIndexSignatureDeclaration);
                     if (node.parameters.length === 1) {
                         const parameter = node.parameters[0];
-                        if (parameter && parameter.type && parameter.type.kind === syntaxKind) {
+                        if (parameter.type && parameter.type.kind === syntaxKind) {
                             return node;
                         }
                     }
@@ -7218,7 +7218,7 @@ namespace ts {
             return undefined;
         }
 
-        function createIndexInfo(type: Type, isReadonly: boolean, declaration?: SignatureDeclaration): IndexInfo {
+        function createIndexInfo(type: Type, isReadonly: boolean, declaration?: IndexSignatureDeclaration): IndexInfo {
             return { type, isReadonly, declaration };
         }
 
diff --git a/src/compiler/types.ts b/src/compiler/types.ts
index f820201063c..d9305c23b3e 100644
--- a/src/compiler/types.ts
+++ b/src/compiler/types.ts
@@ -3968,7 +3968,7 @@ namespace ts {
     export interface IndexInfo {
         type: Type;
         isReadonly: boolean;
-        declaration?: SignatureDeclaration;
+        declaration?: IndexSignatureDeclaration;
     }
 
     /* @internal */
diff --git a/tests/baselines/reference/api/tsserverlibrary.d.ts b/tests/baselines/reference/api/tsserverlibrary.d.ts
index 89c1b54eec5..05ad706c56f 100644
--- a/tests/baselines/reference/api/tsserverlibrary.d.ts
+++ b/tests/baselines/reference/api/tsserverlibrary.d.ts
@@ -2249,7 +2249,7 @@ declare namespace ts {
     interface IndexInfo {
         type: Type;
         isReadonly: boolean;
-        declaration?: SignatureDeclaration;
+        declaration?: IndexSignatureDeclaration;
     }
     enum InferencePriority {
         NakedTypeVariable = 1,
diff --git a/tests/baselines/reference/api/typescript.d.ts b/tests/baselines/reference/api/typescript.d.ts
index dbb4d6cc7bf..2278e8bb509 100644
--- a/tests/baselines/reference/api/typescript.d.ts
+++ b/tests/baselines/reference/api/typescript.d.ts
@@ -2249,7 +2249,7 @@ declare namespace ts {
     interface IndexInfo {
         type: Type;
         isReadonly: boolean;
-        declaration?: SignatureDeclaration;
+        declaration?: IndexSignatureDeclaration;
     }
     enum InferencePriority {
         NakedTypeVariable = 1,