diff --git a/pg/src/main/java/org/bouncycastle/gpg/SExpression.java b/pg/src/main/java/org/bouncycastle/gpg/SExpression.java
index ee5083ad5e..308ef31b53 100644
--- a/pg/src/main/java/org/bouncycastle/gpg/SExpression.java
+++ b/pg/src/main/java/org/bouncycastle/gpg/SExpression.java
@@ -104,6 +104,10 @@ private static SExpression parseExpression(InputStream src, SExpression expr, By
 
                 if (c == ':')
                 {
+                    if (expr == null)
+                    {
+                        throw new IOException("invalid input stream at ':'");
+                    }
                     try
                     {
                         int len = Integer.parseInt(Strings.fromByteArray(accumulator.toByteArray()));
diff --git a/pg/src/test/java/org/bouncycastle/openpgp/test/SExprTest.java b/pg/src/test/java/org/bouncycastle/openpgp/test/SExprTest.java
index 03862f62c7..05c11a2580 100644
--- a/pg/src/test/java/org/bouncycastle/openpgp/test/SExprTest.java
+++ b/pg/src/test/java/org/bouncycastle/openpgp/test/SExprTest.java
@@ -1,15 +1,18 @@
 package org.bouncycastle.openpgp.test;
 
 import java.io.ByteArrayInputStream;
+import java.io.IOException;
 import java.security.Security;
 
 import org.bouncycastle.gpg.SExprParser;
+import org.bouncycastle.gpg.SExpression;
 import org.bouncycastle.jce.provider.BouncyCastleProvider;
 import org.bouncycastle.openpgp.PGPPrivateKey;
 import org.bouncycastle.openpgp.PGPSecretKey;
 import org.bouncycastle.openpgp.operator.jcajce.JcaKeyFingerprintCalculator;
 import org.bouncycastle.openpgp.operator.jcajce.JcaPGPDigestCalculatorProviderBuilder;
 import org.bouncycastle.openpgp.operator.jcajce.JcePBEProtectionRemoverFactory;
+import org.bouncycastle.util.Strings;
 import org.bouncycastle.util.encoders.Base64;
 import org.bouncycastle.util.test.SimpleTest;
 
@@ -139,9 +142,25 @@ public String getName()
         return "SExprTest";
     }
 
+    private void corruptStreamTest()
+        throws Exception
+    {
+        try
+        {
+            SExpression.parse(new ByteArrayInputStream(Strings.toByteArray("2:3abc")), 2);
+            fail("no exception");
+        }
+        catch (IOException e)
+        {
+            isEquals("invalid input stream at ':'", e.getMessage());
+        }
+    }
+
     public void performTest()
         throws Exception
     {
+        corruptStreamTest();
+
         SExprParser parser = new SExprParser(new JcaPGPDigestCalculatorProviderBuilder().build());
 
         PGPSecretKey k1 = parser.parseSecretKey(new ByteArrayInputStream(key1), new JcePBEProtectionRemoverFactory("fred".toCharArray()), new JcaKeyFingerprintCalculator());