diff --git a/src/openflexure_microscope_server/stitching.py b/src/openflexure_microscope_server/stitching.py
index 55b8bc0a..1660944e 100644
--- a/src/openflexure_microscope_server/stitching.py
+++ b/src/openflexure_microscope_server/stitching.py
@@ -26,6 +26,10 @@ DEFAULT_OVERLAP = 0.1
 DEFAULT_RESIZE = 0.5
 
 
+class ExternalSigkillError(ChildProcessError):
+    """Exception called when stitch is killed by an external process calling Sigkill."""
+
+
 class StitcherValidationError(RuntimeError):
     """The stitcher received values that it deems unsafe to create a command from."""
 
@@ -303,8 +307,15 @@ class FinalStitcher(BaseStitcher):
 
         if process.poll() == 0:
             self.logger.info("Stitching complete")
+        elif process.poll() == -9:
+            raise ExternalSigkillError(
+                "Stitching was killed by an external process. This is "
+                "most likely due to running out of memory."
+            )
         else:
-            raise ChildProcessError(f"Subprocess {STITCHING_CMD} exited with an error.")
+            raise ChildProcessError(
+                f"Subprocess {STITCHING_CMD} errored with exit code {process.poll()}."
+            )
 
     def _log_ongoing(
         self,