diff --git a/spec/std/indexable_spec.cr b/spec/std/indexable_spec.cr
index 69d7b2f36a32..4b9a100df430 100644
--- a/spec/std/indexable_spec.cr
+++ b/spec/std/indexable_spec.cr
@@ -61,24 +61,40 @@ private class SafeRecursiveIndexable
 end
 
 describe Indexable do
-  it "does index with big negative offset" do
-    indexable = SafeIndexable.new(3)
-    indexable.index(0, -100).should be_nil
-  end
+  describe "#index" do
+    it "does index with big negative offset" do
+      indexable = SafeIndexable.new(3)
+      indexable.index(0, -100).should be_nil
+    end
 
-  it "does index with big offset" do
-    indexable = SafeIndexable.new(3)
-    indexable.index(0, 100).should be_nil
-  end
+    it "does index with big offset" do
+      indexable = SafeIndexable.new(3)
+      indexable.index(0, 100).should be_nil
+    end
 
-  it "does rindex with big negative offset" do
-    indexable = SafeIndexable.new(3)
-    indexable.rindex(0, -100).should be_nil
+    it "offset type" do
+      indexable = SafeIndexable.new(3)
+      indexable.index(1, 0_i64).should eq 1
+      indexable.index(1, 0_i64).should be_a(Int64)
+    end
   end
 
-  it "does rindex with big offset" do
-    indexable = SafeIndexable.new(3)
-    indexable.rindex(0, 100).should be_nil
+  describe "#rindex" do
+    it "does rindex with big negative offset" do
+      indexable = SafeIndexable.new(3)
+      indexable.rindex(0, -100).should be_nil
+    end
+
+    it "does rindex with big offset" do
+      indexable = SafeIndexable.new(3)
+      indexable.rindex(0, 100).should be_nil
+    end
+
+    it "offset type" do
+      indexable = SafeIndexable.new(3)
+      indexable.rindex(1, 2_i64).should eq 1
+      indexable.rindex(1, 2_i64).should be_a(Int64)
+    end
   end
 
   it "does each" do
diff --git a/spec/std/string_spec.cr b/spec/std/string_spec.cr
index fb6079528127..325b95a550dc 100644
--- a/spec/std/string_spec.cr
+++ b/spec/std/string_spec.cr
@@ -840,7 +840,7 @@ describe "String" do
     it { "hello".presence.should eq("hello") }
   end
 
-  describe "index" do
+  describe "#index" do
     describe "by char" do
       it { "foo".index('o').should eq(1) }
       it { "foo".index('g').should be_nil }
@@ -855,6 +855,12 @@ describe "String" do
         it { "foo".index('g', 1).should be_nil }
         it { "foo".index('g', -20).should be_nil }
         it { "日本語日本語".index('本', 2).should eq(4) }
+
+        # Check offset type
+        it { "foobarbaz".index('a', 5_i64).should eq(7) }
+        it { "foobarbaz".index('a', 5_i64).should be_a(Int32) }
+        it { "日本語日本語".index('本', 2_i64).should eq(4) }
+        it { "日本語日本語".index('本', 2_i64).should be_a(Int32) }
       end
     end
 
@@ -877,6 +883,14 @@ describe "String" do
         it { "日本語日本語".index("本語", 2).should eq(4) }
         it { "\xFD\x9A\xAD\x50NG".index("PNG", 2).should eq(3) }
         it { "\xFD\x9A\xAD\x50NG".index("PNG", 4).should be_nil }
+
+        # Check offset type
+        it { "foobarbaz".index("a", 5_i64).should eq(7) }
+        it { "foobarbaz".index("a", 5_i64).should be_a(Int32) }
+        it { "日本語日本語".index("本", 2_i64).should eq(4) }
+        it { "日本語日本語".index("本", 2_i64).should be_a(Int32) }
+        it { "日本語日本語".index("", 2_i64).should eq 2 }
+        it { "日本語日本語".index("", 2_i64).should be_a(Int64) }
       end
     end
 
@@ -899,7 +913,7 @@ describe "String" do
     end
   end
 
-  describe "rindex" do
+  describe "#rindex" do
     describe "by char" do
       it { "bbbb".rindex('b').should eq(3) }
       it { "foobar".rindex('a').should eq(4) }
@@ -918,6 +932,12 @@ describe "String" do
         it { "faobar".rindex('a', 3).should eq(1) }
         it { "faobarbaz".rindex('a', -3).should eq(4) }
         it { "日本語日本語".rindex('本', 3).should eq(1) }
+
+        # Check offset type
+        it { "bbbb".rindex('b', 2_i64).should eq(2) }
+        it { "bbbb".rindex('b', 2_i64).should be_a(Int64) }
+        it { "日本語日本語".rindex('本', 3_i64).should eq(1) }
+        it { "日本語日本語".rindex('本', 3_i64).should be_a(Int64) }
       end
     end
 
@@ -939,6 +959,14 @@ describe "String" do
         it { "foo".rindex("", 3).should eq(3) }
         it { "foo".rindex("", 4).should eq(3) }
         it { "日本語日本語".rindex("日本", 2).should eq(0) }
+
+        # Check offset type
+        it { "bbbb".rindex("b", 2_i64).should eq(2) }
+        it { "bbbb".rindex("b", 2_i64).should be_a(Int32) }
+        it { "日本語日本語".rindex("本", 3_i64).should eq(1) }
+        it { "日本語日本語".rindex("本", 3_i64).should be_a(Int32) }
+        it { "日本語日本語".rindex("", 3_i64).should eq(3) }
+        it { "日本語日本語".rindex("", 3_i64).should be_a(Int32) }
       end
     end
 
diff --git a/src/array.cr b/src/array.cr
index 2e06885d203a..3b83b49a4ea0 100644
--- a/src/array.cr
+++ b/src/array.cr
@@ -2231,7 +2231,7 @@ class Array(T)
   end
 
   # :nodoc:
-  def index(object, offset : Int = 0) : Int32?
+  def index(object, offset : Int = 0)
     # Optimize for the case of looking for a byte in a byte slice
     if T.is_a?(UInt8.class) &&
        (object.is_a?(UInt8) || (object.is_a?(Int) && 0 <= object < 256))
diff --git a/src/indexable.cr b/src/indexable.cr
index d121ec478027..352a28459ff7 100644
--- a/src/indexable.cr
+++ b/src/indexable.cr
@@ -532,7 +532,7 @@ module Indexable(T)
   # [1, 2, 3, 2, 3].rindex(2)            # => 3
   # [1, 2, 3, 2, 3].rindex(2, offset: 2) # => 1
   # ```
-  def rindex(value, offset = size - 1) : Int32?
+  def rindex(value, offset = size - 1)
     rindex(offset) { |elem| elem == value }
   end
 
diff --git a/src/string.cr b/src/string.cr
index a6c22706344c..caefdae1243b 100644
--- a/src/string.cr
+++ b/src/string.cr
@@ -3035,7 +3035,7 @@ class String
   def index(search : Char, offset = 0) : Int32?
     # If it's ASCII we can delegate to slice
     if search.ascii? && single_byte_optimizable?
-      return to_slice.index(search.ord.to_u8, offset)
+      return to_slice.fast_index(search.ord.to_u8, offset)
     end
 
     offset += size if offset < 0
@@ -3051,7 +3051,7 @@ class String
   end
 
   # :ditto:
-  def index(search : String, offset = 0) : Int32?
+  def index(search : String, offset = 0)
     offset += size if offset < 0
     return if offset < 0
 
@@ -3127,7 +3127,7 @@ class String
   # "Hello, World".rindex("o", 5) # => 4
   # "Hello, World".rindex("W", 2) # => nil
   # ```
-  def rindex(search : Char, offset = size - 1) : Int32?
+  def rindex(search : Char, offset = size - 1)
     # If it's ASCII we can delegate to slice
     if search.ascii? && single_byte_optimizable?
       return to_slice.rindex(search.ord.to_u8, offset)