diff --git a/fava_client.py b/fava_client.py
index c61e1d9..afc964f 100644
--- a/fava_client.py
+++ b/fava_client.py
@@ -44,6 +44,22 @@ def _infer_target_file(account_name: str) -> str:
     return "accounts/chart.beancount"
 
 
+def _escape_beancount_string(value: str) -> str:
+    """Escape a value for safe inclusion in a Beancount string literal.
+
+    Beancount's tokenizer unescapes \\", \\n, \\t, \\r, \\\\ etc. (tokens.c
+    cunescape). Unescaped quotes or newlines in free-text metadata written
+    straight into the ledger source would corrupt the file, so escape the
+    backslash first (to keep it round-tripping) then quotes and newlines.
+    """
+    return (
+        value.replace("\\", "\\\\")
+        .replace('"', '\\"')
+        .replace("\n", "\\n")
+        .replace("\r", "\\r")
+    )
+
+
 class FavaClient:
     """
     Async client for Fava REST API.
@@ -1544,7 +1560,7 @@ class FavaClient:
     async def add_account(
         self,
         account_name: str,
-        currencies: list[str],
+        currencies: Optional[list[str]] = None,
         opening_date: Optional[date] = None,
         metadata: Optional[Dict[str, Any]] = None,
         target_file: Optional[str] = None,
@@ -1642,19 +1658,23 @@ class FavaClient:
                         lines = source.split('\n')
                         insert_index = len(lines)
 
-                        # Step 4: Format Open directive as Beancount text
-                        currencies_str = ", ".join(currencies)
-                        open_lines = [
-                            "",
-                            f"{opening_date.isoformat()} open {account_name} {currencies_str}"
-                        ]
+                        # Step 4: Format Open directive as Beancount text.
+                        # Currencies are an optional constraint on an Open
+                        # directive; when none are given the account accepts
+                        # any commodity.
+                        open_directive = f"{opening_date.isoformat()} open {account_name}"
+                        if currencies:
+                            open_directive += f" {', '.join(currencies)}"
+                        open_lines = ["", open_directive]
 
                         # Add metadata if provided
                         if metadata:
                             for key, value in metadata.items():
                                 # Format metadata with proper indentation
                                 if isinstance(value, str):
-                                    open_lines.append(f'  {key}: "{value}"')
+                                    open_lines.append(
+                                        f'  {key}: "{_escape_beancount_string(value)}"'
+                                    )
                                 else:
                                     open_lines.append(f'  {key}: {value}')