Update app.py

app.py

@@ -104,8 +104,13 @@ def validate_excel_structure(df):
         validation_messages.append("Invalid format in Publication Year column")
     
     # Check if DOIs are in valid format (basic check)
-    if not df['DOI'].str.contains(r'10\.\d{4,}/.+', na=True).all():
-        validation_messages.append("Some DOIs are in invalid format")
+    try:
+        # Convert DOI column to string and handle NaN values
+        doi_series = df['DOI'].fillna('')
+        if not doi_series.astype(str).str.contains(r'10\.\d{4,}/.+', na=True).all():
+            validation_messages.append("Some DOIs are in invalid format or missing")
+    except Exception as e:
+        validation_messages.append(f"Error validating DOI format: {str(e)}")
     
     return len(validation_messages) == 0, validation_messages