diff --git a/src/fs.cr b/src/fs.cr
index 23f4df7..7c549ac 100644
--- a/src/fs.cr
+++ b/src/fs.cr
@@ -104,9 +104,13 @@ class FS::Hash(K, V)
 	end
 
 	def []=(key : K, value : V)
+		old_value = self.[key]?
+
+		check_collisions! key, value, old_value
+
 		# Removes any old indices or partitions pointing to a value about
 		# to be replaced.
-		self.[key]?.try do |old_value|
+		if old_value
 			remove_partitions key, old_value
 		end
 
@@ -119,16 +123,36 @@ class FS::Hash(K, V)
 		write_partitions key, value
 	end
 
+	def check_collisions!(key : K, value : V, old_value : V?)
+		@partitions.each do |index|
+			case index
+			when IndexData
+				index_key = index.key_proc.call value
+
+				symlink = file_path_indexes(index_key.to_s, index.name)
+				# FIXME: Check it’s not pointing to “old_value”, if any.
+
+				pp! symlink
+
+				if ::File.exists? symlink
+					raise IndexOverload.new "Index '#{index.name}' is overloaded for key '#{key}'"
+				end
+			end
+		end
+	end
+
 	def write_partitions(key : K, value : V)
 		@partitions.each do |index|
 			index_key = index.key_proc.call value
 
 			case index
 			when IndexData
-				symlink = file_path_indexes(key.to_s, index.name)
+				symlink = file_path_indexes(index_key, index.name)
 
 				Dir.mkdir_p ::File.dirname symlink
 
+				# FIXME: A check_collisions! is done a bit higher. Is this
+				#        still required?
 				if ::File.exists? symlink
 					raise Exception.new "symlink already exists: #{symlink}"
 				end
@@ -183,7 +207,7 @@ class FS::Hash(K, V)
 
 			case index
 			when IndexData
-				symlink = file_path_indexes(key.to_s, index.name)
+				symlink = file_path_indexes(index_key, index.name)
 
 				::File.delete symlink
 			when PartitionData
@@ -260,8 +284,8 @@ class FS::Hash(K, V)
 		"#{@directory_name}/tags/by_#{name}"
 	end
 
-	private def file_path_indexes(key : String, index_name : String)
-		"#{dir_path_indexes index_name}/#{key}.json"
+	private def file_path_indexes(index_key : String, index_name : String)
+		"#{dir_path_indexes index_name}/#{index_key}.json"
 	end
 
 	private def file_path_partition(key : String, index_name : String, index_key : String)
@@ -289,3 +313,6 @@ class FS::Hash(K, V)
 	end
 end
 
+class FS::IndexOverload < Exception
+end
+
diff --git a/test-index.cr b/test-index.cr
new file mode 100644
index 0000000..3d14723
--- /dev/null
+++ b/test-index.cr
@@ -0,0 +1,44 @@
+require "json"
+require "./src/fs.cr"
+require "uuid"
+
+# This test basically works if no data is obtained when fetching "broken"
+# partitions/indices/tags.
+
+class Ship
+	JSON.mapping({
+		id: String,
+		class: String,
+		name: String,
+		tags: Array(String)
+	})
+
+	def initialize(@name, @class = @name, @tags = [] of String)
+		@id = UUID.random.to_s
+	end
+
+	getter name
+	getter id
+end
+
+ships = FS::Hash(String, Ship).new "test-index"
+by_name   = ships.new_index        "name", &.name
+by_class  = ships.new_partition    "class", &.class
+by_id     = ships.new_index        "id",   &.id
+by_tags   = ships.new_tags         "tags", &.tags
+
+ship = Ship.new "Mutsuki", "Mutsuki", tags: ["kuchikukan"]
+ships[ship.id] = ship
+
+begin
+	ship = Ship.new "Mutsuki", "broken", tags: ["kuchikukan"]
+	ships[ship.id] = ship
+rescue FS::IndexOverload
+	puts "rescue: Adding an entry that would overload an index has been prevented."
+	# Should happen, ignore it.
+else
+	puts "ERROR: No IndexOverload exception was raised on index overload."
+end
+
+pp! ships.get_index("name").map &.name
+