diff --git a/docs/api.rst b/docs/api.rst
index af1aa4d1..d0ad6b54 100644
--- a/docs/api.rst
+++ b/docs/api.rst
@@ -650,7 +650,7 @@ All objects from ``attrs.converters`` are also available from ``attr.converters`
       C(x='')
 
 
-.. autofunction:: attrs.converters.to_bool
+.. autofunction:: attrs.converters.to_bool(val)
 
    For example:
 
@@ -665,10 +665,11 @@ All objects from ``attrs.converters`` are also available from ``attr.converters`
       C(x=True)
       >>> C(0)
       C(x=False)
-      >>> C("foo")
+      >>> C("norway")
       Traceback (most recent call last):
          File "<stdin>", line 1, in <module>
-      ValueError: Cannot convert value to bool: foo
+      ValueError: Cannot convert value to bool: norway
+
 
 
 
diff --git a/src/attr/converters.py b/src/attr/converters.py
index 30b9af94..bfe8a23e 100644
--- a/src/attr/converters.py
+++ b/src/attr/converters.py
@@ -131,15 +131,14 @@ def to_bool(val):
     """
     if isinstance(val, str):
         val = val.lower()
-    truthy = {True, "true", "t", "yes", "y", "on", "1", 1}
-    falsy = {False, "false", "f", "no", "n", "off", "0", 0}
     try:
-        if val in truthy:
+        if val in (True, "true", "t", "yes", "y", "on", "1", 1):
             return True
-        if val in falsy:
+        if val in (False, "false", "f", "no", "n", "off", "0", 0):
             return False
     except TypeError:
         # Raised when "val" is not hashable (e.g., lists)
         pass
-    msg = f"Cannot convert value to bool: {val}"
+
+    msg = f"Cannot convert value to bool: {val!r}"
     raise ValueError(msg)