diff --git a/tools/ci.sh b/tools/ci.sh
index efa492bd19..975ce86172 100755
--- a/tools/ci.sh
+++ b/tools/ci.sh
@@ -44,7 +44,9 @@ function ci_code_spell_run {
 
 function ci_commit_formatting_run {
     git remote add upstream https://github.com/micropython/micropython.git
-    git fetch --depth=100 upstream  master
+    git fetch --depth=100 upstream master
+    # If the common ancestor commit hasn't been found, fetch more.
+    git merge-base upstream/master HEAD || git fetch --unshallow upstream master
     # For a PR, upstream/master..HEAD ends with a merge commit into master, exclude that one.
     tools/verifygitlog.py -v upstream/master..HEAD --no-merges
 }
@@ -68,6 +70,8 @@ function ci_code_size_build {
     git checkout -b pull_request # save the current location
     git remote add upstream https://github.com/micropython/micropython.git
     git fetch --depth=100 upstream master
+    # If the common ancestor commit hasn't been found, fetch more.
+    git merge-base upstream/master HEAD || git fetch --unshallow upstream master
     # build reference, save to size0
     # ignore any errors with this build, in case master is failing
     git checkout `git merge-base --fork-point upstream/master pull_request`