diff --git a/spec/std/enumerable_spec.cr b/spec/std/enumerable_spec.cr
index 084fe80dcf96..baffa410110e 100644
--- a/spec/std/enumerable_spec.cr
+++ b/spec/std/enumerable_spec.cr
@@ -1,4 +1,5 @@
 require "spec"
+require "../spec_helper"
 require "spec/helpers/iterate"
 
 module SomeInterface; end
@@ -1364,6 +1365,19 @@ describe "Enumerable" do
     it { [1, 2, 3].sum(4.5).should eq(10.5) }
     it { (1..3).sum { |x| x * 2 }.should eq(12) }
     it { (1..3).sum(1.5) { |x| x * 2 }.should eq(13.5) }
+    it { [1, 3_u64].sum(0_i32).should eq(4_u32) }
+    it { [1, 3].sum(0_u64).should eq(4_u64) }
+    it { [1, 10000000000_u64].sum(0_u64).should eq(10000000001) }
+    it "raises if union types are summed", tags: %w[slow] do
+      exc = assert_error <<-CRYSTAL,
+        require "prelude"
+        [1, 10000000000_u64].sum
+        CRYSTAL
+        "`Enumerable#sum()` and `#product()` do not support Union " +
+        "types. Instead, use `Enumerable#sum(initial)` and " +
+        "`#product(initial)`, respectively, with an initial value " +
+        "of the intended type of the call."
+    end
 
     it "uses additive_identity from type" do
       typeof([1, 2, 3].sum).should eq(Int32)
@@ -1405,6 +1419,20 @@ describe "Enumerable" do
       typeof([1.5, 2.5, 3.5].product).should eq(Float64)
       typeof([1, 2, 3].product(&.to_f)).should eq(Float64)
     end
+
+    it { [1, 3_u64].product(3_i32).should eq(9_u32) }
+    it { [1, 3].product(3_u64).should eq(9_u64) }
+    it { [1, 10000000000_u64].product(3_u64).should eq(30000000000_u64) }
+    it "raises if union types are multiplied", tags: %w[slow] do
+      exc = assert_error <<-CRYSTAL,
+        require "prelude"
+        [1, 10000000000_u64].product
+        CRYSTAL
+        "`Enumerable#sum()` and `#product()` do not support Union " +
+        "types. Instead, use `Enumerable#sum(initial)` and " +
+        "`#product(initial)`, respectively, with an initial value " +
+        "of the intended type of the call."
+    end
   end
 
   describe "first" do
diff --git a/src/enumerable.cr b/src/enumerable.cr
index 0993f38bbc4d..f341409dc66c 100644
--- a/src/enumerable.cr
+++ b/src/enumerable.cr
@@ -1771,7 +1771,7 @@ module Enumerable(T)
   end
 
   private def additive_identity(reflect)
-    type = reflect.first
+    type = reflect.type
     if type.responds_to? :additive_identity
       type.additive_identity
     else
@@ -1808,7 +1808,10 @@ module Enumerable(T)
   # Expects all types returned from the block to respond to `#+` method.
   #
   # This method calls `.additive_identity` on the yielded type to determine the
-  # type of the sum value.
+  # type of the sum value.  Hence, it can fail to compile if
+  # `.additive_identity` fails to determine a safe type, e.g., in case of
+  # union types.  In such cases, use `sum(initial)` with an initial value of
+  # the expected type of the sum value.
   #
   # If the collection is empty, returns `additive_identity`.
   #
@@ -1847,7 +1850,7 @@ module Enumerable(T)
   # ```
   #
   # This method calls `.multiplicative_identity` on the element type to determine the
-  # type of the sum value.
+  # type of the product value.
   #
   # If the collection is empty, returns `multiplicative_identity`.
   #
@@ -1855,7 +1858,7 @@ module Enumerable(T)
   # ([] of Int32).product # => 1
   # ```
   def product
-    product Reflect(T).first.multiplicative_identity
+    product Reflect(T).type.multiplicative_identity
   end
 
   # Multiplies *initial* and all the elements in the collection
@@ -1886,8 +1889,11 @@ module Enumerable(T)
   #
   # Expects all types returned from the block to respond to `#*` method.
   #
-  # This method calls `.multiplicative_identity` on the element type to determine the
-  # type of the sum value.
+  # This method calls `.multiplicative_identity` on the element type to
+  # determine the type of the product value.  Hence, it can fail to compile if
+  # `.multiplicative_identity` fails to determine a safe type, e.g., in case
+  # of union types.  In such cases, use `product(initial)` with an initial
+  # value of the expected type of the product value.
   #
   # If the collection is empty, returns `multiplicative_identity`.
   #
@@ -1895,7 +1901,7 @@ module Enumerable(T)
   # ([] of Int32).product { |x| x + 1 } # => 1
   # ```
   def product(& : T -> _)
-    product(Reflect(typeof(yield Enumerable.element_type(self))).first.multiplicative_identity) do |value|
+    product(Reflect(typeof(yield Enumerable.element_type(self))).type.multiplicative_identity) do |value|
       yield value
     end
   end
@@ -2287,12 +2293,16 @@ module Enumerable(T)
 
   # :nodoc:
   private struct Reflect(X)
-    # For now it's just a way to implement `Enumerable#sum` in a way that the
-    # initial value given to it has the type of the first type in the union,
-    # if the type is a union.
-    def self.first
+    # For now, Reflect is used to reject union types in `#sum()` and
+    # `#product()` methods.
+    def self.type
       {% if X.union? %}
-        {{X.union_types.first}}
+        {{
+          raise("`Enumerable#sum()` and `#product()` do not support Union " +
+                "types. Instead, use `Enumerable#sum(initial)` and " +
+                "`#product(initial)`, respectively, with an initial value " +
+                "of the intended type of the call.")
+        }}
       {% else %}
         X
       {% end %}