@@ -14,10 +14,12 @@
  * file.
  */
 
 #import "OFObject.h"
 
+OF_ASSUME_NONNULL_BEGIN
+
 @class OFString;
 #ifndef DOXYGEN
 @class OFArray OF_GENERIC(ObjectType);
 @class OFMutableArray OF_GENERIC(ObjectType);
 #endif
@@ -59,11 +61,11 @@
  * the first key/value pair found is returned.
  *
  * @param key The key for which the string value should be returned
  * @return The string value for the specified key, or nil if it does not exist
  */
-- (OFString*)stringForKey: (OFString*)key;
+- (nullable OFString*)stringForKey: (OFString*)key;
 
 /*!
  * @brief Returns the string value for the specified key or the specified
  *	  default value if it does not exist.
  *
@@ -73,12 +75,12 @@
  * @param key The key for which the string value should be returned
  * @param defaultValue The value to return if the key does not exist
  * @return The string value for the specified key or the specified default
  *	   value if it does not exist
  */
-- (OFString*)stringForKey: (OFString*)key
-	     defaultValue: (OFString*)defaultValue;
+- (nullable OFString*)stringForKey: (OFString*)key
+		      defaultValue: (nullable OFString*)defaultValue;
 
 /*!
  * @brief Returns the integer value for the specified key or the specified
  *	  default value if it does not exist.
  *
@@ -234,5 +236,7 @@
  *
  * @param key The key of the value to remove
  */
 - (void)removeValueForKey: (OFString*)key;
 @end
+
+OF_ASSUME_NONNULL_END