diff --git a/org.eclipse.jgit.test/tst-rsrc/org/eclipse/jgit/diff/very_long_file.patch b/org.eclipse.jgit.test/tst-rsrc/org/eclipse/jgit/diff/very_long_file.patch
new file mode 100644
index 000000000..64f2b0068
Binary files /dev/null and b/org.eclipse.jgit.test/tst-rsrc/org/eclipse/jgit/diff/very_long_file.patch differ
diff --git a/org.eclipse.jgit.test/tst-rsrc/org/eclipse/jgit/diff/very_long_file_PostImage b/org.eclipse.jgit.test/tst-rsrc/org/eclipse/jgit/diff/very_long_file_PostImage
new file mode 100644
index 000000000..72ef58f79
Binary files /dev/null and b/org.eclipse.jgit.test/tst-rsrc/org/eclipse/jgit/diff/very_long_file_PostImage differ
diff --git a/org.eclipse.jgit.test/tst-rsrc/org/eclipse/jgit/diff/very_long_file_PreImage b/org.eclipse.jgit.test/tst-rsrc/org/eclipse/jgit/diff/very_long_file_PreImage
new file mode 100644
index 000000000..379a33cf2
Binary files /dev/null and b/org.eclipse.jgit.test/tst-rsrc/org/eclipse/jgit/diff/very_long_file_PreImage differ
diff --git a/org.eclipse.jgit.test/tst/org/eclipse/jgit/patch/PatchApplierTest.java b/org.eclipse.jgit.test/tst/org/eclipse/jgit/patch/PatchApplierTest.java
index 92d47c296..2aac15bbb 100644
--- a/org.eclipse.jgit.test/tst/org/eclipse/jgit/patch/PatchApplierTest.java
+++ b/org.eclipse.jgit.test/tst/org/eclipse/jgit/patch/PatchApplierTest.java
@@ -496,6 +496,14 @@ public void testRemoveNewlineAtEnd() throws Exception {
 			Result result = applyPatch();
 			verifyChange(result, "x_last_rm_nl");
 		}
+
+		@Test
+		public void testVeryLongFile() throws Exception {
+			init("very_long_file");
+
+			Result result = applyPatch();
+			verifyChange(result, "very_long_file");
+		}
 	}
 
 	public static class WithWorktree extends Base {
diff --git a/org.eclipse.jgit/src/org/eclipse/jgit/patch/PatchApplier.java b/org.eclipse.jgit/src/org/eclipse/jgit/patch/PatchApplier.java
index 1edd85421..a327095c8 100644
--- a/org.eclipse.jgit/src/org/eclipse/jgit/patch/PatchApplier.java
+++ b/org.eclipse.jgit/src/org/eclipse/jgit/patch/PatchApplier.java
@@ -88,6 +88,7 @@
 import org.eclipse.jgit.util.TemporaryBuffer.LocalFile;
 import org.eclipse.jgit.util.io.BinaryDeltaInputStream;
 import org.eclipse.jgit.util.io.BinaryHunkInputStream;
+import org.eclipse.jgit.util.io.CountingOutputStream;
 import org.eclipse.jgit.util.io.EolStreamTypeUtil;
 import org.eclipse.jgit.util.sha1.SHA1;
 
@@ -1013,7 +1014,10 @@ && canApplyAt(hunkLines, newLines, 0)) {
 		// We could check if old == new, but the short-circuiting complicates
 		// logic for inCore patching, so just write the new thing regardless.
 		TemporaryBuffer buffer = new TemporaryBuffer.LocalFile(null);
-		try (OutputStream out = buffer) {
+		// TemporaryBuffer::length reports incorrect length until the buffer
+		// is closed. To use it as input for ContentStreamLoader below, we
+		// need a wrapper with a reliable in-progress length.
+		try (CountingOutputStream out = new CountingOutputStream(buffer)) {
 			for (Iterator<ByteBuffer> l = newLines.iterator(); l.hasNext();) {
 				ByteBuffer line = l.next();
 				if (line == null) {
@@ -1026,7 +1030,7 @@ && canApplyAt(hunkLines, newLines, 0)) {
 				}
 			}
 			return new ContentStreamLoader(buffer::openInputStream,
-					buffer.length());
+				out.getCount());
 		}
 	}