Ensure blur reports the received value for invalid sigma

lib/operation.mjs

@@ -379,12 +379,10 @@ function median (size) {
  * @throws {Error} Invalid parameters
  */
 function blur (options) {
-  let sigma;
-  if (is.number(options)) {
-    sigma = options;
-  } else if (is.plainObject(options)) {
+  let sigma = options;
+  if (is.plainObject(options)) {
     if (!is.number(options.sigma)) {
-      throw is.invalidParameterError('options.sigma', 'number between 0.3 and 1000', sigma);
+      throw is.invalidParameterError('options.sigma', 'number between 0.3 and 1000', options.sigma);
     }
     sigma = options.sigma;
     if ('precision' in options) {

test/unit/blur.js

@@ -151,4 +151,18 @@ suite('Blur', () => {
       sharp(fixtures.inputJpg).blur({ precision: 'invalid' });
     }, /Expected number between 0.3 and 1000 for options.sigma but received undefined of type undefined/);
   });
+
+  test('invalid options.sigma', (t) => {
+    t.plan(1);
+    t.assert.throws(() => {
+      sharp(fixtures.inputJpg).blur({ sigma: 'x' });
+    }, /Expected number between 0.3 and 1000 for options.sigma but received x of type string/);
+  });
+
+  test('invalid sigma', (t) => {
+    t.plan(1);
+    t.assert.throws(() => {
+      sharp(fixtures.inputJpg).blur('5');
+    }, /Expected number between 0.3 and 1000 for sigma but received 5 of type string/);
+  });
 });