diff --git a/src/test/java/org/apache/commons/text/StringSubstitutorTest.java b/src/test/java/org/apache/commons/text/StringSubstitutorTest.java
index 0f1fbf4339..3692653f10 100644
--- a/src/test/java/org/apache/commons/text/StringSubstitutorTest.java
+++ b/src/test/java/org/apache/commons/text/StringSubstitutorTest.java
@@ -1078,4 +1078,11 @@ void testSubstitutePreserveEscape() throws IOException {
         assertEqualsCharSeq("value $${escaped}", replace(sub, org));
     }
 
+    @Test
+    void testDetectsCyclicSubstitution() {
+        final Map<String, String> map = new HashMap<>();
+        map.put("name", "<name>");
+        assertThrows(IllegalStateException.class, () -> StringSubstitutor.replace("Hi <name>!", map, "<", ">"));
+    }
+
 }
diff --git a/src/test/java/org/apache/commons/text/similarity/HammingDistanceTest.java b/src/test/java/org/apache/commons/text/similarity/HammingDistanceTest.java
index 181d2fe277..918a7bedc2 100644
--- a/src/test/java/org/apache/commons/text/similarity/HammingDistanceTest.java
+++ b/src/test/java/org/apache/commons/text/similarity/HammingDistanceTest.java
@@ -59,6 +59,21 @@ void testHammingDistance_nullRightValue() {
         assertThrows(IllegalArgumentException.class, () -> distance.apply("", null));
     }
 
+        @Test
+    void testApply_NullSimilarityInput() {
+        assertThrows(IllegalArgumentException.class, () -> distance.apply(null, new SimilarityCharacterInput("a")));
+    }
+
+    @Test
+    void testApply_SimilarityInputNull() {
+        assertThrows(IllegalArgumentException.class, () -> distance.apply(new SimilarityCharacterInput("a"), null));
+    }
+
+    @Test
+    void testApply_DifferentSimilarityInputLength() {
+        assertThrows(IllegalArgumentException.class, () -> distance.apply(new SimilarityCharacterInput("a"), new SimilarityCharacterInput("ab")));
+    }
+
     @Test
     void testHammingDistanceCharSequence() {
         assertEquals(0, distance.apply("", ""));
diff --git a/src/test/java/org/apache/commons/text/similarity/JaroWinklerDistanceTest.java b/src/test/java/org/apache/commons/text/similarity/JaroWinklerDistanceTest.java
index f1242fb301..412f1e721c 100644
--- a/src/test/java/org/apache/commons/text/similarity/JaroWinklerDistanceTest.java
+++ b/src/test/java/org/apache/commons/text/similarity/JaroWinklerDistanceTest.java
@@ -95,4 +95,14 @@ void testGetJaroWinklerDistance_StringString() {
         assertEquals(1 - 0.51111d, distance.apply("foo", "  foo"), 0.00001d);
     }
 
+    @Test
+    void testApply_NullSimilarityInput() {
+        assertThrows(IllegalArgumentException.class, () -> distance.apply(null, new SimilarityCharacterInput("a")));
+    }
+
+    @Test
+    void testApply_SimilarityInputNull() {
+        assertThrows(IllegalArgumentException.class, () -> distance.apply(new SimilarityCharacterInput("a"), null));
+    }
+
 }
diff --git a/src/test/java/org/apache/commons/text/similarity/JaroWinklerSimilarityTest.java b/src/test/java/org/apache/commons/text/similarity/JaroWinklerSimilarityTest.java
index a5acfb2394..0b1f97052a 100644
--- a/src/test/java/org/apache/commons/text/similarity/JaroWinklerSimilarityTest.java
+++ b/src/test/java/org/apache/commons/text/similarity/JaroWinklerSimilarityTest.java
@@ -136,4 +136,14 @@ void testGetJaroWinklerSimilarity_StringString() {
         assertEquals(0.911111d, similarity.apply(wrap("John Horn"), "John Hopkins"), 0.00001d);
     }
 
+        @Test
+    void testApply_NullSimilarityInput() {
+        assertThrows(IllegalArgumentException.class, () -> similarity.apply(null, new SimilarityCharacterInput("a")));
+    }
+
+    @Test
+    void testApply_SimilarityInputNull() {
+        assertThrows(IllegalArgumentException.class, () -> similarity.apply(new SimilarityCharacterInput("a"), null));
+    }
+
 }