diff --git a/.flowconfig b/.flowconfig
index 0aafcc683b..fa9472bb98 100644
--- a/.flowconfig
+++ b/.flowconfig
@@ -11,7 +11,7 @@
 
 [lints]
 sketchy-null-bool=off
-sketchy-null-string=off
+sketchy-null-string=error
 sketchy-null-number=error
 sketchy-null-mixed=error
 sketchy-number=error
diff --git a/src/jsutils/invariant.js b/src/jsutils/invariant.js
index d82cd87a19..a53db54a8e 100644
--- a/src/jsutils/invariant.js
+++ b/src/jsutils/invariant.js
@@ -3,6 +3,8 @@
 export default function invariant(condition: mixed, message?: string): void {
   const booleanCondition = Boolean(condition);
   if (!booleanCondition) {
-    throw new Error(message || 'Unexpected invariant triggered');
+    throw new Error(
+      message != null ? message : 'Unexpected invariant triggered',
+    );
   }
 }
diff --git a/src/language/source.js b/src/language/source.js
index b7c31ef464..2e2127a377 100644
--- a/src/language/source.js
+++ b/src/language/source.js
@@ -23,7 +23,7 @@ export class Source {
 
   constructor(body: string, name?: string, locationOffset?: Location): void {
     this.body = body;
-    this.name = name || 'GraphQL request';
+    this.name = name != null ? name : 'GraphQL request';
     this.locationOffset = locationOffset || { line: 1, column: 1 };
     devAssert(
       this.locationOffset.line > 0,
diff --git a/src/utilities/buildClientSchema.js b/src/utilities/buildClientSchema.js
index 366da69625..0ccccbb59d 100644
--- a/src/utilities/buildClientSchema.js
+++ b/src/utilities/buildClientSchema.js
@@ -350,9 +350,10 @@ export function buildClientSchema(
 
   function buildInputValue(inputValueIntrospection) {
     const type = getInputType(inputValueIntrospection.type);
-    const defaultValue = inputValueIntrospection.defaultValue
-      ? valueFromAST(parseValue(inputValueIntrospection.defaultValue), type)
-      : undefined;
+    const defaultValue =
+      inputValueIntrospection.defaultValue != null
+        ? valueFromAST(parseValue(inputValueIntrospection.defaultValue), type)
+        : undefined;
     return {
       description: inputValueIntrospection.description,
       type,
diff --git a/src/utilities/extendSchema.js b/src/utilities/extendSchema.js
index 35bc1067ad..f803c8437c 100644
--- a/src/utilities/extendSchema.js
+++ b/src/utilities/extendSchema.js
@@ -222,7 +222,7 @@ export function extendSchema(
   }
 
   function getMaybeTypeByName(typeName: ?string): ?GraphQLNamedType {
-    return typeName ? typeMap[typeName] : null;
+    return typeName != null ? typeMap[typeName] : null;
   }
 
   function getMergedDirectives(): Array<GraphQLDirective> {
diff --git a/src/utilities/findDeprecatedUsages.js b/src/utilities/findDeprecatedUsages.js
index 694937ff58..120680d132 100644
--- a/src/utilities/findDeprecatedUsages.js
+++ b/src/utilities/findDeprecatedUsages.js
@@ -34,7 +34,7 @@ export function findDeprecatedUsages(
             errors.push(
               new GraphQLError(
                 `The field "${parentType.name}.${fieldDef.name}" is deprecated.` +
-                  (reason ? ' ' + reason : ''),
+                  (reason != null ? ' ' + reason : ''),
                 node,
               ),
             );
@@ -50,7 +50,7 @@ export function findDeprecatedUsages(
             errors.push(
               new GraphQLError(
                 `The enum value "${type.name}.${enumVal.name}" is deprecated.` +
-                  (reason ? ' ' + reason : ''),
+                  (reason != null && reason !== '' ? ' ' + reason : ''),
                 node,
               ),
             );
diff --git a/src/utilities/getOperationAST.js b/src/utilities/getOperationAST.js
index 5cb9986eb3..6b34b6dcea 100644
--- a/src/utilities/getOperationAST.js
+++ b/src/utilities/getOperationAST.js
@@ -18,7 +18,7 @@ export function getOperationAST(
   let operation = null;
   for (const definition of documentAST.definitions) {
     if (definition.kind === Kind.OPERATION_DEFINITION) {
-      if (!operationName) {
+      if (operationName == null) {
         // If no operation name was provided, only return an Operation if there
         // is one defined in the document. Upon encountering the second, return
         // null.