diff --git a/gems/smithy-cbor/lib/smithy-cbor/parser.rb b/gems/smithy-cbor/lib/smithy-cbor/parser.rb
index d59889e6a..6401e599d 100644
--- a/gems/smithy-cbor/lib/smithy-cbor/parser.rb
+++ b/gems/smithy-cbor/lib/smithy-cbor/parser.rb
@@ -35,7 +35,7 @@ def list(shape, values, result = nil)
         sparse = target.traits.key?('smithy.api#sparse')
         result = [] if result.nil?
         values.each do |value|
-          next if value.nil? && !sparse
+          raise ParseError, "Expected #{list_member.target.name}, got nil" if value.nil? && !sparse
 
           result << parse_shape(list_member, value)
         end
@@ -48,7 +48,7 @@ def map(shape, values, result = nil)
         sparse = target.traits.key?('smithy.api#sparse')
         result = {} if result.nil?
         values.each do |key, value|
-          next if value.nil? && !sparse
+          raise ParseError, "Expected #{value_member.target.name}, got nil" if value.nil? && !sparse
 
           result[key] = parse_shape(value_member, value)
         end
diff --git a/gems/smithy-cbor/spec/smithy-cbor/parser_spec.rb b/gems/smithy-cbor/spec/smithy-cbor/parser_spec.rb
index 860c5d83b..52564d9fb 100644
--- a/gems/smithy-cbor/spec/smithy-cbor/parser_spec.rb
+++ b/gems/smithy-cbor/spec/smithy-cbor/parser_spec.rb
@@ -114,10 +114,11 @@ module Cbor
           expect(subject.parse(structure_shape, bytes).to_h).to eq(list: ['string'])
         end
 
-        it 'parses lists with nil values' do
+        it 'rejects nil values in non-sparse lists' do
           data = { 'list' => [nil] }
           bytes = Cbor.encode(data)
-          expect(subject.parse(structure_shape, bytes).to_h).to eq(list: [])
+          expect { subject.parse(structure_shape, bytes) }
+            .to raise_error(Cbor::ParseError, 'Expected String, got nil')
         end
 
         it 'parses sparse lists' do
@@ -135,10 +136,11 @@ module Cbor
           expect(subject.parse(structure_shape, bytes).to_h).to eq(map: { 'key' => 'value' })
         end
 
-        it 'parses maps with nil values' do
+        it 'rejects nil values in non-sparse maps' do
           data = { 'map' => { 'key' => nil } }
           bytes = Cbor.encode(data)
-          expect(subject.parse(structure_shape, bytes).to_h).to eq(map: {})
+          expect { subject.parse(structure_shape, bytes) }
+            .to raise_error(Cbor::ParseError, 'Expected String, got nil')
         end
 
         it 'parses sparse maps' do
diff --git a/gems/smithy-json/lib/smithy-json/parser.rb b/gems/smithy-json/lib/smithy-json/parser.rb
index 706d1b9b0..ea96ed90a 100644
--- a/gems/smithy-json/lib/smithy-json/parser.rb
+++ b/gems/smithy-json/lib/smithy-json/parser.rb
@@ -50,7 +50,7 @@ def list(shape, values, result = nil)
         sparse = target.traits.key?('smithy.api#sparse')
         result = [] if result.nil?
         values.each do |value|
-          next if value.nil? && !sparse
+          raise ParseError, "Expected #{member.target.name}, got nil" if value.nil? && !sparse
 
           result << parse_shape(member, value)
         end
@@ -63,7 +63,7 @@ def map(shape, values, result = nil)
         sparse = target.traits.key?('smithy.api#sparse')
         result = {} if result.nil?
         values.each do |key, value|
-          next if value.nil? && !sparse
+          raise ParseError, "Expected #{value_member.target.name}, got nil" if value.nil? && !sparse
 
           result[key] = parse_shape(value_member, value)
         end
diff --git a/gems/smithy-json/spec/smithy-json/parser_spec.rb b/gems/smithy-json/spec/smithy-json/parser_spec.rb
index 37bbe15e1..93a9d3d2c 100644
--- a/gems/smithy-json/spec/smithy-json/parser_spec.rb
+++ b/gems/smithy-json/spec/smithy-json/parser_spec.rb
@@ -123,7 +123,8 @@ module Json
         it 'parses unknown members' do
           data = { union: { 'someThing' => 'someValue' } }
           bytes = Json.dump(data)
-          expect(subject.parse(structure_shape, bytes).to_h).to eq(union: { unknown: { 'someThing' => 'someValue' } })
+          expect(subject.parse(structure_shape, bytes).to_h)
+            .to eq(union: { unknown: { 'someThing' => 'someValue' } })
         end
 
         it 'parsing ignores an extra __type key' do
@@ -152,10 +153,11 @@ module Json
           expect(subject.parse(structure_shape, bytes).to_h).to eq(list: ['string'])
         end
 
-        it 'parses lists with nil values' do
+        it 'rejects nil values in non-sparse lists' do
           data = { 'list' => [nil] }
           bytes = Json.dump(data)
-          expect(subject.parse(structure_shape, bytes).to_h).to eq(list: [])
+          expect { subject.parse(structure_shape, bytes) }
+            .to raise_error(Json::ParseError, 'Expected String, got nil')
         end
 
         it 'parses sparse lists' do
@@ -173,17 +175,19 @@ module Json
           expect(subject.parse(structure_shape, bytes).to_h).to eq(map: { 'key' => 'value' })
         end
 
-        it 'parses maps with nil values' do
+        it 'rejects nil values in non-sparse maps' do
           data = { 'map' => { 'key' => nil } }
           bytes = Json.dump(data)
-          expect(subject.parse(structure_shape, bytes).to_h).to eq(map: {})
+          expect { subject.parse(structure_shape, bytes) }
+            .to raise_error(Json::ParseError, 'Expected String, got nil')
         end
 
         it 'parses sparse maps' do
           shapes['smithy.ruby.tests#Map']['traits'] = { 'smithy.api#sparse' => {} }
           data = { 'map' => { 'key' => nil, 'anotherKey' => 'value' } }
           bytes = Json.dump(data)
-          expect(subject.parse(structure_shape, bytes).to_h).to eq(map: { 'key' => nil, 'anotherKey' => 'value' })
+          expect(subject.parse(structure_shape, bytes).to_h)
+            .to eq(map: { 'key' => nil, 'anotherKey' => 'value' })
         end
       end
 
@@ -243,7 +247,8 @@ module Json
         it 'handles unrecognized timestamp formats' do
           data = { 'timestamp' => 'unrecognized format' }
           bytes = Json.dump(data)
-          expect { subject.parse(structure_shape, bytes) }.to raise_error(/unhandled epoch-seconds timestamp/)
+          expect { subject.parse(structure_shape, bytes) }
+            .to raise_error(/unhandled epoch-seconds timestamp/)
         end
       end
     end