diff --git a/.flowconfig b/.flowconfig
index 385a9dde28..75fcf47dbe 100644
--- a/.flowconfig
+++ b/.flowconfig
@@ -14,7 +14,7 @@ sketchy-null-bool=off
 sketchy-null-string=off
 sketchy-null-number=error
 sketchy-null-mixed=off
-sketchy-number=off
+sketchy-number=error
 untyped-type-import=error
 nonstrict-import=off
 untyped-import=off
diff --git a/src/validation/__tests__/PossibleTypeExtensions-test.js b/src/validation/__tests__/PossibleTypeExtensions-test.js
index b140f5e8ad..115f5c57bc 100644
--- a/src/validation/__tests__/PossibleTypeExtensions-test.js
+++ b/src/validation/__tests__/PossibleTypeExtensions-test.js
@@ -35,7 +35,7 @@ function extendingDifferentTypeKind(typeName, kind, l1, c1, l2, c2) {
   const message = extendingDifferentTypeKindMessage(typeName, kind);
   const locations = [{ line: l1, column: c1 }];
 
-  if (l2 && c2) {
+  if (l2 !== undefined && c2 !== undefined) {
     locations.push({ line: l2, column: c2 });
   }
   return { message, locations };