diff --git a/src/Smalot/PdfParser/Font.php b/src/Smalot/PdfParser/Font.php
index b6207bb8..2e05cff5 100644
--- a/src/Smalot/PdfParser/Font.php
+++ b/src/Smalot/PdfParser/Font.php
@@ -283,7 +283,9 @@ public function calculateTextWidth(string $text, array &$missing = null): ?float
     {
         $index_map = array_flip($this->table);
         $details = $this->getDetails();
-        $widths = $details['Widths'];
+
+        // Usually, Widths key is set in $details array, but if it isn't use an empty array instead.
+        $widths = $details['Widths'] ?? [];
 
         // Widths array is zero indexed but table is not. We must map them based on FirstChar and LastChar
         $width_map = array_flip(range($details['FirstChar'], $details['LastChar']));
diff --git a/tests/PHPUnit/Integration/FontTest.php b/tests/PHPUnit/Integration/FontTest.php
index c1aeabba..e76a051d 100644
--- a/tests/PHPUnit/Integration/FontTest.php
+++ b/tests/PHPUnit/Integration/FontTest.php
@@ -454,6 +454,29 @@ public function testCalculateTextWidth(): void
         $this->assertEquals([], $missing);
     }
 
+    /**
+     * Check behavior if getDetails() does return an array without a Widths-key.
+     *
+     * @see https://github.com/smalot/pdfparser/issues/619
+     */
+    public function testCalculateTextWidthNoWidthsKey(): void
+    {
+        $document = $this->createMock(Document::class);
+
+        $header = $this->createMock(Header::class);
+        $header->method('getDetails')->willReturn([
+            'FirstChar' => '',
+            'LastChar' => '',
+            // 'Widths' key is not set, so without the fix in Font.php a warning would be thrown.
+        ]);
+
+        $font = new Font($document, $header);
+        $font->setTable([]);
+        $width = $font->calculateTextWidth('foo');
+
+        $this->assertNull($width);
+    }
+
     /**
      * Check behavior if iconv function gets input which contains illegal characters.
      *