diff --git a/tests/unit/capi/GEOSRelatePatternTest.cpp b/tests/unit/capi/GEOSRelatePatternTest.cpp
index 1b6068749f..819e6ad79f 100644
--- a/tests/unit/capi/GEOSRelatePatternTest.cpp
+++ b/tests/unit/capi/GEOSRelatePatternTest.cpp
@@ -33,7 +33,7 @@ void object::test<1>()
 
 template<>
 template<>
-void object::test<11>()
+void object::test<2>()
 {
     geom1_ = fromWKT("CIRCULARSTRING (0 0, 1 1, 2 0)");
     geom2_ = fromWKT("LINESTRING (1 0, 2 1)");
@@ -45,5 +45,26 @@ void object::test<11>()
     ensure_equals("curved geometry not supported", GEOSRelatePattern(geom2_, geom1_, "0********"), 2);
 }
 
+// invalid DE-9IM
+template<>
+template<>
+void object::test<3>()
+{
+    geom1_ = fromWKT("POINT(1 2)");
+    geom2_ = fromWKT("POINT(1 2)");
+
+    ensure(geom1_);
+    ensure(geom2_);
+
+    // pattern too long
+    ensure_equals(2, GEOSRelatePattern(geom1_, geom2_, "0FFFFF2120000000000000000000"));
+
+    // pattern too short
+    ensure_equals(2, GEOSRelatePattern(geom1_, geom2_, "0F"));
+
+    // pattern has invalid characters
+    ensure_equals(0, GEOSRelatePattern(geom1_, geom2_, "123456789"));
+}
+
 } // namespace tut