diff --git a/src/compiler/checker.ts b/src/compiler/checker.ts
index 0ccf23164cb..2932de8e99b 100644
--- a/src/compiler/checker.ts
+++ b/src/compiler/checker.ts
@@ -10250,25 +10250,25 @@ namespace ts {
             // constructors of derived classes must contain at least one super call somewhere in their function body.
             let containingClassDecl = <ClassDeclaration>node.parent;
             if (getClassExtendsHeritageClauseElement(containingClassDecl)) {
-                let symbol = getSymbolOfNode(containingClassDecl);
-                let type = <InterfaceType>getDeclaredTypeOfSymbol(symbol);
-                let baseConstructorType = getBaseConstructorTypeOfClass(type);
+                let containingClassSymbol = getSymbolOfNode(containingClassDecl);
+                let containingClassInstanceType = <InterfaceType>getDeclaredTypeOfSymbol(containingClassSymbol);
+                let baseConstructorType = getBaseConstructorTypeOfClass(containingClassInstanceType);
 
                 if (containsSuperCall(node.body)) {
                     if (baseConstructorType === nullType) {
-                        error(node, Diagnostics.A_constructor_cannot_contain_super_call_when_a_class_extends_null);
+                        error(node, Diagnostics.A_constructor_cannot_contain_a_super_call_when_its_class_extends_null);
                     }
 
                     // The first statement in the body of a constructor must be a super call if both of the following are true:
                     // - The containing class is a derived class.
                     // - The constructor declares parameter properties
                     //   or the containing class declares instance member variables with initializers.
-                    let statements = (<Block>node.body).statements;
                     let superCallShouldBeFirst =
                         forEach((<ClassDeclaration>node.parent).members, isInstancePropertyWithInitializer) ||
                         forEach(node.parameters, p => p.flags & (NodeFlags.Public | NodeFlags.Private | NodeFlags.Protected));
 
                     if (superCallShouldBeFirst) {
+                        let statements = (<Block>node.body).statements;
                         if (!statements.length || statements[0].kind !== SyntaxKind.ExpressionStatement || !isSuperCallExpression((<ExpressionStatement>statements[0]).expression)) {
                             error(node, Diagnostics.A_super_call_must_be_the_first_statement_in_the_constructor_when_a_class_contains_initialized_properties_or_has_parameter_properties);
                         }
diff --git a/src/compiler/diagnosticInformationMap.generated.ts b/src/compiler/diagnosticInformationMap.generated.ts
index 44f14ac0995..cdfe28eb781 100644
--- a/src/compiler/diagnosticInformationMap.generated.ts
+++ b/src/compiler/diagnosticInformationMap.generated.ts
@@ -613,6 +613,6 @@ namespace ts {
         Expected_corresponding_JSX_closing_tag_for_0: { code: 17002, category: DiagnosticCategory.Error, key: "Expected corresponding JSX closing tag for '{0}'." },
         JSX_attribute_expected: { code: 17003, category: DiagnosticCategory.Error, key: "JSX attribute expected." },
         Cannot_use_JSX_unless_the_jsx_flag_is_provided: { code: 17004, category: DiagnosticCategory.Error, key: "Cannot use JSX unless the '--jsx' flag is provided." },
-        A_constructor_cannot_contain_super_call_when_a_class_extends_null: { code: 17005, category: DiagnosticCategory.Error, key: "A constructor cannot contain 'super' call when a class extends 'null'" },
+        A_constructor_cannot_contain_a_super_call_when_its_class_extends_null: { code: 17005, category: DiagnosticCategory.Error, key: "A constructor cannot contain a 'super' call when its class extends 'null'" },
     };
 }
\ No newline at end of file
diff --git a/src/compiler/diagnosticMessages.json b/src/compiler/diagnosticMessages.json
index 1203c5ef61e..d0023ce6cfd 100644
--- a/src/compiler/diagnosticMessages.json
+++ b/src/compiler/diagnosticMessages.json
@@ -1,4 +1,4 @@
-{
+ {
     "Unterminated string literal.": {
         "category": "Error",
         "code": 1002
@@ -2446,7 +2446,7 @@
         "category": "Error",
         "code": 17004
     },
-    "A constructor cannot contain 'super' call when a class extends 'null'": {
+    "A constructor cannot contain a 'super' call when its class extends 'null'": {
       "category": "Error",
       "code": 17005
     }
diff --git a/tests/baselines/reference/classExtendsNull.errors.txt b/tests/baselines/reference/classExtendsNull.errors.txt
index cc5de5cda83..7bb44774826 100644
--- a/tests/baselines/reference/classExtendsNull.errors.txt
+++ b/tests/baselines/reference/classExtendsNull.errors.txt
@@ -1,4 +1,4 @@
-tests/cases/compiler/classExtendsNull.ts(2,5): error TS17005: A constructor cannot contain 'super' call when a class extends 'null'
+tests/cases/compiler/classExtendsNull.ts(2,5): error TS17005: A constructor cannot contain a 'super' call when its class extends 'null'
 
 
 ==== tests/cases/compiler/classExtendsNull.ts (1 errors) ====
@@ -11,7 +11,7 @@ tests/cases/compiler/classExtendsNull.ts(2,5): error TS17005: A constructor cann
     ~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
         }
     ~~~~~
-!!! error TS17005: A constructor cannot contain 'super' call when a class extends 'null'
+!!! error TS17005: A constructor cannot contain a 'super' call when its class extends 'null'
     }
     
     class D extends null {