diff --git a/src/benchmark/bm.cr b/src/benchmark/bm.cr
index 4b94e4ca6eee..4036c23e8d35 100644
--- a/src/benchmark/bm.cr
+++ b/src/benchmark/bm.cr
@@ -44,13 +44,13 @@ module Benchmark
       end
 
       # Reports a single benchmark unit.
-      def report(label = " ", &block)
+      def report(label : String = " ", &block : ->) : Nil
         @label_width = label.size if label.size > @label_width
         @reports << {label, block}
       end
 
       # :nodoc:
-      def execute
+      def execute : Nil
         if @label_width > 0
           print " " * @label_width
         end
diff --git a/src/benchmark/ips.cr b/src/benchmark/ips.cr
index def5b09c7c66..a78d67d184d4 100644
--- a/src/benchmark/ips.cr
+++ b/src/benchmark/ips.cr
@@ -31,7 +31,7 @@ module Benchmark
       end
 
       # Adds code to be benchmarked
-      def report(label = "", &action) : Benchmark::IPS::Entry
+      def report(label : String = "", &action : ->) : Benchmark::IPS::Entry
         item = Entry.new(label, action)
         @items << item
         item