diff --git a/src/camera.js b/src/camera.js
index 8041a9e..cf20f73 100644
--- a/src/camera.js
+++ b/src/camera.js
@@ -15,7 +15,14 @@ export async function startCamera(videoElement) {
 
   videoElement.srcObject = stream;
   videoElement.setAttribute('playsinline', '');
-  await videoElement.play();
+
+  try {
+    await videoElement.play();
+  } catch (error) {
+    for (const track of stream.getTracks()) track.stop();
+    videoElement.srcObject = null;
+    throw error;
+  }
 
   return {
     stop() {
@@ -38,8 +45,17 @@ function drawScaled(source, sourceWidth, sourceHeight, maxEdge) {
   return context.getImageData(0, 0, width, height);
 }
 
+function assertHasSize(width, height) {
+  if (!(width > 0) || !(height > 0)) {
+    throw new Error(
+      'Das Kamerabild ist noch nicht bereit. Bitte gleich noch einmal versuchen.',
+    );
+  }
+}
+
 /** Einzelbild aus dem laufenden Kamerastrom. */
 export function grabFrame(videoElement, maxEdge = 1280) {
+  assertHasSize(videoElement.videoWidth, videoElement.videoHeight);
   return drawScaled(
     videoElement,
     videoElement.videoWidth,
@@ -51,6 +67,7 @@ export function grabFrame(videoElement, maxEdge = 1280) {
 /** Ersatzweg ohne Kamera: Bilddatei auswaehlen (z. B. am Rechner). */
 export async function imageDataFromFile(file, maxEdge = 1280) {
   const bitmap = await createImageBitmap(file);
+  assertHasSize(bitmap.width, bitmap.height);
   const data = drawScaled(bitmap, bitmap.width, bitmap.height, maxEdge);
   bitmap.close();
   return data;