diff --git a/spec/std/enumerable_spec.cr b/spec/std/enumerable_spec.cr
index 084fe80dcf96..78c7fa8bc7da 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) }
+    pending_wasm32 "raises if union types are summed", tags: %w[slow] do
+      assert_compile_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) }
+    pending_wasm32 "raises if union types are multiplied", tags: %w[slow] do
+      assert_compile_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/spec/std/spec_helper.cr b/spec/std/spec_helper.cr
index 8bb0a9e1a2f2..f0c03d05ece5 100644
--- a/spec/std/spec_helper.cr
+++ b/spec/std/spec_helper.cr
@@ -101,6 +101,33 @@ def compile_file(source_file, *, bin_name = "executable_file", flags = %w(), fil
   end
 end
 
+def assert_compile_error(source, expected_error, *, flags = %w(), file = __FILE__, line = __LINE__)
+  # can't use backtick in interpreted code (#12241)
+  pending_interpreted! "Unable to compile Crystal code in interpreted code"
+
+  with_tempfile("source_file", file: file) do |source_file|
+    File.write(source_file, source)
+
+    bin_name = "executable_file"
+    with_temp_executable(bin_name, file: file) do |executable_file|
+      compiler = ENV["CRYSTAL_SPEC_COMPILER_BIN"]? || "bin/crystal"
+      args = ["build"] + flags + ["-o", executable_file, source_file]
+      output = IO::Memory.new
+      status = Process.run(compiler, args, env: {
+        "CRYSTAL_PATH"         => Crystal::PATH,
+        "CRYSTAL_LIBRARY_PATH" => Crystal::LIBRARY_PATH,
+        "CRYSTAL_CACHE_DIR"    => Crystal::CACHE_DIR,
+        "NO_COLOR"             => "1",
+      }, output: output, error: output)
+
+      output.to_s.should contain(expected_error)
+
+      status.success?.should be_false
+      File.exists?(executable_file).should be_false
+    end
+  end
+end
+
 def compile_source(source, flags = %w(), file = __FILE__, &)
   with_tempfile("source_file", file: file) do |source_file|
     File.write(source_file, source)
diff --git a/spec/support/wasm32.cr b/spec/support/wasm32.cr
index 98c9de69ea9e..9beeaa43fb7e 100644
--- a/spec/support/wasm32.cr
+++ b/spec/support/wasm32.cr
@@ -1,16 +1,16 @@
 require "spec"
 
 {% if flag?(:wasm32) %}
-  def pending_wasm32(description = "assert", file = __FILE__, line = __LINE__, end_line = __END_LINE__, &block)
-    pending("#{description} [wasm32]", file, line, end_line)
+  def pending_wasm32(description = "assert", file = __FILE__, line = __LINE__, end_line = __END_LINE__, focus : Bool = false, tags : String | Enumerable(String) | Nil = nil, &block)
+    pending("#{description} [wasm32]", file, line, end_line, focus: focus, tags: tags)
   end
 
   def pending_wasm32(*, describe, file = __FILE__, line = __LINE__, end_line = __END_LINE__, &block)
     pending_wasm32(describe, file, line, end_line) { }
   end
 {% else %}
-  def pending_wasm32(description = "assert", file = __FILE__, line = __LINE__, end_line = __END_LINE__, &block)
-    it(description, file, line, end_line, &block)
+  def pending_wasm32(description = "assert", file = __FILE__, line = __LINE__, end_line = __END_LINE__, focus : Bool = false, tags : String | Enumerable(String) | Nil = nil, &block)
+    it(description, file, line, end_line, focus: focus, tags: tags, &block)
   end
 
   def pending_wasm32(*, describe, file = __FILE__, line = __LINE__, end_line = __END_LINE__, &block)
diff --git a/src/enumerable.cr b/src/enumerable.cr
index 9fcba66ddf3a..99c004997b95 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
@@ -2285,12 +2291,16 @@ 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 %}