diff --git a/org.eclipse.jgit.test/tst/org/eclipse/jgit/util/io/UnionInputStreamTest.java b/org.eclipse.jgit.test/tst/org/eclipse/jgit/util/io/UnionInputStreamTest.java
index 9fb323c4d..daebffbc1 100644
--- a/org.eclipse.jgit.test/tst/org/eclipse/jgit/util/io/UnionInputStreamTest.java
+++ b/org.eclipse.jgit.test/tst/org/eclipse/jgit/util/io/UnionInputStreamTest.java
@@ -50,6 +50,7 @@
 
 import java.io.ByteArrayInputStream;
 import java.io.IOException;
+import java.io.InputStream;
 import java.util.Arrays;
 
 import org.junit.Test;
@@ -238,4 +239,26 @@ public void close() throws IOException {
 			assertEquals("I AM A TEST", e.getMessage());
 		}
 	}
+
+	@Test
+	public void testNonBlockingPartialRead() throws Exception {
+		InputStream errorReadStream = new InputStream() {
+			@Override
+			public int read() throws IOException {
+				throw new IOException("Expected");
+			}
+		};
+		final UnionInputStream u = new UnionInputStream(
+				new ByteArrayInputStream(new byte[]{1,2,3}),
+				errorReadStream);
+		byte buf[] = new byte[10];
+		assertEquals(3, u.read(buf, 0, 10));
+		assertTrue(Arrays.equals(new byte[] {1,2,3}, slice(buf, 3)));
+		try {
+			u.read(buf, 0, 1);
+			fail("Expected exception from errorReadStream");
+		} catch (IOException e) {
+			assertEquals("Expected", e.getMessage());
+		}
+	}
 }
diff --git a/org.eclipse.jgit/src/org/eclipse/jgit/util/io/UnionInputStream.java b/org.eclipse.jgit/src/org/eclipse/jgit/util/io/UnionInputStream.java
index 0df3775f1..20dcd4659 100644
--- a/org.eclipse.jgit/src/org/eclipse/jgit/util/io/UnionInputStream.java
+++ b/org.eclipse.jgit/src/org/eclipse/jgit/util/io/UnionInputStream.java
@@ -138,23 +138,18 @@ else if (in == EOF)
 
 	@Override
 	public int read(byte[] b, int off, int len) throws IOException {
-		int cnt = 0;
-		while (0 < len) {
+		if (len == 0)
+			return 0;
+		for (;;) {
 			final InputStream in = head();
 			final int n = in.read(b, off, len);
-			if (0 < n) {
-				cnt += n;
-				off += n;
-				len -= n;
-			} else if (in == EOF)
-				return 0 < cnt ? cnt : -1;
-			else {
+			if (0 < n)
+				return n;
+			else if (in == EOF)
+				return -1;
+			else
 				pop();
-				if (0 < cnt)
-					break;
-			}
 		}
-		return cnt;
 	}
 
 	@Override