diff --git a/dist/index.js b/dist/index.js
index 01d3178..6ec828a 100644
--- a/dist/index.js
+++ b/dist/index.js
@@ -37923,13 +37923,18 @@ let BASE_SHA;
                     process.stdout.write('\n');
                     process.stdout.write(`NOTE: You can instead make this a hard error by setting 'error-on-no-successful-workflow' on the action in your workflow.\n`);
                     process.stdout.write('\n');
-                    const commitCountOutput = (0, child_process_1.spawnSync)('git', ['rev-list', '--count', `origin/${mainBranchName}`], { encoding: 'utf-8' }).stdout;
-                    const commitCount = parseInt(stripNewLineEndings(commitCountOutput), 10);
-                    const LAST_COMMIT_CMD = `origin/${mainBranchName}${commitCount > 1 ? '~1' : ''}`;
+                    // Check if HEAD~1 exists, and if not, set BASE_SHA to undefined
+                    const LAST_COMMIT_CMD = `origin/${mainBranchName}~1`;
                     const baseRes = (0, child_process_1.spawnSync)('git', ['rev-parse', LAST_COMMIT_CMD], {
                         encoding: 'utf-8',
                     });
-                    BASE_SHA = baseRes.stdout;
+                    if (baseRes.status !== 0 || !baseRes.stdout) {
+                        process.stdout.write(`HEAD~1 does not exist. BASE_SHA is set to undefined.\n`);
+                        BASE_SHA = undefined;
+                    }
+                    else {
+                        BASE_SHA = baseRes.stdout;
+                    }
                 }
                 core.setOutput('noPreviousBuild', 'true');
             }
diff --git a/find-successful-workflow.ts b/find-successful-workflow.ts
index c2c1324..b9b4e66 100644
--- a/find-successful-workflow.ts
+++ b/find-successful-workflow.ts
@@ -94,23 +94,21 @@ let BASE_SHA: string;
           );
           process.stdout.write('\n');
 
-          const commitCountOutput = spawnSync(
-            'git',
-            ['rev-list', '--count', `origin/${mainBranchName}`],
-            { encoding: 'utf-8' },
-          ).stdout;
-          const commitCount = parseInt(
-            stripNewLineEndings(commitCountOutput),
-            10,
-          );
+          // Check if HEAD~1 exists, and if not, set BASE_SHA to undefined
+          const LAST_COMMIT_CMD = `origin/${mainBranchName}~1`;
 
-          const LAST_COMMIT_CMD = `origin/${mainBranchName}${
-            commitCount > 1 ? '~1' : ''
-          }`;
           const baseRes = spawnSync('git', ['rev-parse', LAST_COMMIT_CMD], {
             encoding: 'utf-8',
           });
-          BASE_SHA = baseRes.stdout;
+
+          if (baseRes.status !== 0 || !baseRes.stdout) {
+            process.stdout.write(
+              `HEAD~1 does not exist. BASE_SHA is set to undefined.\n`,
+            );
+            BASE_SHA = undefined;
+          } else {
+            BASE_SHA = baseRes.stdout;
+          }
         }
         core.setOutput('noPreviousBuild', 'true');
       }