diff --git a/action.yml b/action.yml
index 9655516..cc1ef9c 100644
--- a/action.yml
+++ b/action.yml
@@ -55,10 +55,10 @@ inputs:
     default: ${{ github.event.pull_request.head.sha }}
     required: false
 
-  pull_request_base:
-    description: "The commit right before the pull-request commits (intenral)"
-    default: ${{ github.event.pull_request.base.sha }}
-    required: true
+  pull_request_commits:
+    description: "Number of commits in this PR"
+    required: false
+    default: ${{ github.event.commits }}
 
 outputs:
   sarif:
diff --git a/container/cmd b/container/cmd
index 72bde58..4b2b4f5 100755
--- a/container/cmd
+++ b/container/cmd
@@ -42,7 +42,6 @@ set_linter_options()
     for tag in $INPUT_LINTER_TAGS; do
         linter_options+=( --linter-tag "$tag" )
     done
-    linter_options+=( --compare-against "$INPUT_PULL_REQUEST_BASE" )
 }
 
 populate_sarif_data()
@@ -89,6 +88,10 @@ if test "$GITHUB_EVENT_NAME" = 'pull_request'; then
     # We don't want to rebase on top of the current upstream's head; the line
     # numbers would be shifted and the provided output would be misleading.
 
+    # Get the last commit before this PR.
+    last_committed=$(git rev-parse "HEAD~$INPUT_PULL_REQUEST_COMMITS")
+    linter_options+=( --compare-against "$last_committed" )
+
     test -z "$INPUT_SUBDIRECTORY" || INPUT_SUBDIRECTORIES=$INPUT_SUBDIRECTORY
     test -z "$INPUT_SUBDIRECTORIES" && INPUT_SUBDIRECTORIES=.