diff --git a/buster/busterbot.py b/buster/busterbot.py
index 90a80cd..51d8d2a 100644
--- a/buster/busterbot.py
+++ b/buster/busterbot.py
@@ -70,7 +70,7 @@ def __init__(self, retriever: Retriever, completer: Completer, validator: Valida
         self.retriever = retriever
         self.validator = validator
 
-    def process_input(self, user_input: str, source: str = "") -> Completion:
+    def process_input(self, user_input: str, source: str = None) -> Completion:
         """
         Main function to process the input question and generate a formatted output.
         """
diff --git a/buster/retriever/base.py b/buster/retriever/base.py
index e2d89a9..7a8f03d 100644
--- a/buster/retriever/base.py
+++ b/buster/retriever/base.py
@@ -21,7 +21,7 @@ def __init__(self, top_k, thresh, max_tokens, embedding_model, *args, **kwargs):
         self.embedding_model = embedding_model
 
     @abstractmethod
-    def get_documents(self, source: str) -> pd.DataFrame:
+    def get_documents(self, source: str = None) -> pd.DataFrame:
         """Get all current documents from a given source."""
         ...
 
diff --git a/buster/retriever/service.py b/buster/retriever/service.py
index 6cdc51f..4a0d83b 100644
--- a/buster/retriever/service.py
+++ b/buster/retriever/service.py
@@ -32,13 +32,28 @@ def __init__(
         self.db = self.client[mongo_db_name]
 
     def get_source_id(self, source: str) -> str:
-        """Get the id of a source."""
-        return str(self.db.sources.find_one({"name": source})["_id"])
+        """Get the id of a source. Returns empty string if the source does not exist."""
+        source_pointer = self.db.sources.find_one({"name": source})
+        return "" if source_pointer is None else str(source_pointer["_id"])
 
-    def get_documents(self, source: str) -> pd.DataFrame:
-        """Get all current documents from a given source."""
-        source_id = self.get_source_id(source)
-        return self.db.documents.find({"source_id": source_id})
+    def get_documents(self, source: str = None) -> pd.DataFrame:
+        """Get all current documents from a given source.
+
+        If source is None, returns all documents. If source does not exist, returns empty dataframe."""
+
+        if source is None:
+            # No source specified, return all documents
+            documents = self.db.documents.find()
+        else:
+            assert isinstance(source, str), "source must be a valid string."
+            source_id = self.get_source_id(source)
+
+            if source_id == "":
+                logger.warning(f"{source=} not found.")
+
+            documents = self.db.documents.find({"source_id": source_id})
+
+        return pd.DataFrame(list(documents))
 
     def get_source_display_name(self, source: str) -> str:
         """Get the display name of a source."""
@@ -52,7 +67,7 @@ def retrieve(self, query: str, top_k: int = None, source: str = None) -> pd.Data
         if top_k is None:
             # use default top_k value
             top_k = self.top_k
-        if source is "" or source is None:
+        if source == "" or source is None:
             filter = None
         else:
             filter = {"source": {"$eq": source}}
diff --git a/buster/retriever/sqlite.py b/buster/retriever/sqlite.py
index fbcec3c..c2f3996 100644
--- a/buster/retriever/sqlite.py
+++ b/buster/retriever/sqlite.py
@@ -44,10 +44,10 @@ def __del__(self):
         if self.db_path is not None:
             self.conn.close()
 
-    def get_documents(self, source: str) -> pd.DataFrame:
+    def get_documents(self, source: str = None) -> pd.DataFrame:
         """Get all current documents from a given source."""
         # Execute the SQL statement and fetch the results.
-        if source == "":
+        if source is None:
             results = self.conn.execute("SELECT * FROM documents")
         else:
             results = self.conn.execute("SELECT * FROM documents WHERE source = ?", (source,))