diff --git a/src/components/RedocStandalone.tsx b/src/components/RedocStandalone.tsx
index 41805c8d..fb241e39 100644
--- a/src/components/RedocStandalone.tsx
+++ b/src/components/RedocStandalone.tsx
@@ -16,6 +16,26 @@ export interface RedocStandaloneProps {
 }
 
 export class RedocStandalone extends React.Component<RedocStandaloneProps> {
+  static propTypes = {
+    spec: (props, _, componentName) => {
+      if (!props.spec && !props.specUrl) {
+        return new Error(
+          `One of props 'spec' or 'specUrlurl' was not specified in '${componentName}'.`,
+        );
+      }
+      return null;
+    },
+
+    specUrl: (props, _, componentName) => {
+      if (!props.spec && !props.specUrl) {
+        return new Error(
+          `One of props 'spec' or 'specUrl' was not specified in '${componentName}'.`,
+        );
+      }
+      return null;
+    },
+  };
+
   render() {
     const { spec, specUrl, options } = this.props;
 
diff --git a/src/standalone.tsx b/src/standalone.tsx
index ecf063cb..16500b34 100644
--- a/src/standalone.tsx
+++ b/src/standalone.tsx
@@ -33,7 +33,9 @@ export function init(
     throw new Error('"element" argument is not provided and <redoc> tag is not found on the page');
   }
 
-  let specUrl, spec;
+  let specUrl: string | undefined;
+  let spec: object | undefined;
+
   if (typeof specOrSpecUrl === 'string') {
     specUrl = specOrSpecUrl;
   } else if (typeof specOrSpecUrl === 'object') {