diff --git a/src/compiler/checker.ts b/src/compiler/checker.ts
index 2b815c40aba..425d149db39 100644
--- a/src/compiler/checker.ts
+++ b/src/compiler/checker.ts
@@ -21865,7 +21865,7 @@ namespace ts {
                     diagnosticMessage = Diagnostics.Octal_literal_types_must_use_ES2015_syntax_Use_the_syntax_0;
                 }
                 if (diagnosticMessage) {
-                    const withMinus = isMinusPrefixUnaryExpression(node.parent);
+                    const withMinus = isPrefixUnaryExpression(node.parent) && node.parent.operator === SyntaxKind.MinusToken;
                     const literal = `${withMinus ? "-" : ""}0o${node.text}`;
                     return grammarErrorOnNode(withMinus ? node.parent : node, diagnosticMessage, literal);
                 }
diff --git a/src/compiler/utilities.ts b/src/compiler/utilities.ts
index f272805ced2..edfbe639d25 100644
--- a/src/compiler/utilities.ts
+++ b/src/compiler/utilities.ts
@@ -742,9 +742,8 @@ namespace ts {
         return false;
     }
 
-    export function isMinusPrefixUnaryExpression(node: Node): boolean {
-        return node.kind === SyntaxKind.PrefixUnaryExpression &&
-            (node as PrefixUnaryExpression).operator === SyntaxKind.MinusToken;
+    export function isPrefixUnaryExpression(node: Node): node is PrefixUnaryExpression {
+        return node.kind === SyntaxKind.PrefixUnaryExpression;
     }
 
     // Warning: This has the same semantics as the forEach family of functions,