diff --git a/core/src/main/java/com/google/bitcoin/script/ScriptBuilder.java b/core/src/main/java/com/google/bitcoin/script/ScriptBuilder.java
index ffac76bd..08b1d1c8 100644
--- a/core/src/main/java/com/google/bitcoin/script/ScriptBuilder.java
+++ b/core/src/main/java/com/google/bitcoin/script/ScriptBuilder.java
@@ -203,7 +203,8 @@ public class ScriptBuilder {
     /**
      * Returns a copy of the given scriptSig with the signature inserted in the given position.
      *
-     * This function assumes that any missing sigs have OP_0 placeholders.
+     * This function assumes that any missing sigs have OP_0 placeholders. If given scriptSig already has all the signatures
+     * in place, IllegalArgumentException will be thrown.
      *
      * @param targetIndex where to insert the signature
      * @param sigsPrefixCount how many items to copy verbatim (e.g. initial OP_0 for multisig)
@@ -215,6 +216,12 @@ public class ScriptBuilder {
         List<ScriptChunk> inputChunks = scriptSig.getChunks();
         int totalChunks = inputChunks.size();
 
+        // Check if we have a place to insert, otherwise just return given scriptSig unchanged.
+        // We assume here that OP_0 placeholders always go after the sigs, so
+        // to find if we have sigs missing, we can just check the chunk in latest sig position
+        boolean hasMissingSigs = inputChunks.get(totalChunks - sigsSuffixCount - 1).equalsOpCode(OP_0);
+        checkArgument(hasMissingSigs, "ScriptSig is already filled with signatures");
+
         // copy the prefix
         for (ScriptChunk chunk: inputChunks.subList(0, sigsPrefixCount))
             builder.addChunk(chunk);
diff --git a/core/src/test/java/com/google/bitcoin/script/ScriptTest.java b/core/src/test/java/com/google/bitcoin/script/ScriptTest.java
index f15f48a8..36e79f79 100644
--- a/core/src/test/java/com/google/bitcoin/script/ScriptTest.java
+++ b/core/src/test/java/com/google/bitcoin/script/ScriptTest.java
@@ -153,7 +153,7 @@ public class ScriptTest {
     }
 
     @Test
-    public void testCreateEmptyInputScript() throws Exception {
+    public void createAndUpdateEmptyInputScript() throws Exception {
         TransactionSignature dummySig = TransactionSignature.dummy();
         ECKey key = new ECKey();
 
@@ -196,6 +196,14 @@ public class ScriptTest {
         assertThat(inputScript.getChunks().get(1).data, equalTo(dummySig.encodeToBitcoin()));
         assertThat(inputScript.getChunks().get(2).data, equalTo(dummySig.encodeToBitcoin()));
         assertThat(inputScript.getChunks().get(3).data, equalTo(multisigScript.getProgram()));
+
+        // updating scriptSig with no missing signatures
+        try {
+            ScriptBuilder.updateScriptWithSignature(inputScript, dummySig.encodeToBitcoin(), 1, 1, 1);
+            fail("Exception expected");
+        } catch (Exception e) {
+            assertEquals(IllegalArgumentException.class, e.getClass());
+        }
     }
 
     private Script parseScriptString(String string) throws IOException {