diff --git a/src/Grok.Net.Tests/Patterns/grok-custom-patterns b/src/Grok.Net.Tests/Patterns/grok-custom-patterns
index a4545c2..897a22e 100644
--- a/src/Grok.Net.Tests/Patterns/grok-custom-patterns
+++ b/src/Grok.Net.Tests/Patterns/grok-custom-patterns
@@ -1 +1,3 @@
-ZIPCODE [1-9]{1}[0-9]{2}\s{0,1}[0-9]{3}
\ No newline at end of file
+ZIPCODE [1-9]{1}[0-9]{2}\s{0,1}[0-9]{3}
+WRONGPATTERN1 \\\\\
+WRONGPATTERN2 \\\\\
diff --git a/src/Grok.Net.Tests/UnitTests.cs b/src/Grok.Net.Tests/UnitTests.cs
index 18a6088..11fe6b4 100644
--- a/src/Grok.Net.Tests/UnitTests.cs
+++ b/src/Grok.Net.Tests/UnitTests.cs
@@ -189,5 +189,30 @@ public void LoadCustomPatterns(string zipcode)
             Assert.Equal(zipcode, grokResult[0].Value);
             Assert.Equal(email, grokResult[1].Value);
         }
+
+        [Fact]
+        public void LoadWrongCustomPatterns()
+        {
+            // Arrange
+            const string client = "192.168.1.1";
+            const string duration = "1";
+
+            var sut = new Grok("%{WRONGPATTERN1:duration}:%{WRONGPATTERN2:client}");
+
+            try
+            {
+                // Act
+                var grokResult = sut.Parse($"{duration}:{client}");
+
+                // Assert (checks if regex is invalid)
+                Assert.Equal("", grokResult[0].Value);
+                Assert.Equal("", grokResult[1].Value);
+            }
+            catch
+            {
+                // Assert (checks if pattern is invalid)
+                Assert.Throws<System.FormatException>(() => sut.Parse($"{duration}:{client}"));
+            }
+        }
     }
 }
diff --git a/src/Grok.Net/Grok.cs b/src/Grok.Net/Grok.cs
index ede9b4f..98ba8db 100644
--- a/src/Grok.Net/Grok.cs
+++ b/src/Grok.Net/Grok.cs
@@ -158,10 +158,23 @@ private void ProcessPatternLine(string line)
             }
 
             string[] strArray = line.Split(new[] { ' ' }, 2);
+            if (strArray.Length != 2)
+            {
+                throw new FormatException("Custom pattern was not in a correct form");
+            }
+
             if (strArray[0].Equals("#", StringComparison.OrdinalIgnoreCase))
             {
                 return;
             }
+            try
+            {
+                Regex.Match("", strArray[1]);
+            }
+            catch
+            {
+                return;
+            }
 
             // check before adding to avoid an exception in case the same pattern is present in the custom patterns file.
             if (!_patterns.ContainsKey(strArray[0]))