diff --git a/modules/swagger-core/src/main/java/io/swagger/v3/core/jackson/ModelResolver.java b/modules/swagger-core/src/main/java/io/swagger/v3/core/jackson/ModelResolver.java
index f25f99a654..e24c358b91 100644
--- a/modules/swagger-core/src/main/java/io/swagger/v3/core/jackson/ModelResolver.java
+++ b/modules/swagger-core/src/main/java/io/swagger/v3/core/jackson/ModelResolver.java
@@ -95,7 +95,6 @@
 import java.math.BigDecimal;
 import java.util.ArrayList;
 import java.util.Arrays;
-import java.util.Collection;
 import java.util.Collections;
 import java.util.HashMap;
 import java.util.HashSet;
@@ -1461,7 +1460,18 @@ protected boolean ignore(final Annotated member, final XmlAccessorType xmlAccess
     private void handleUnwrapped(List<Schema> props, Schema innerModel, String prefix, String suffix, List<String> requiredProps) {
         if (StringUtils.isBlank(suffix) && StringUtils.isBlank(prefix)) {
             if (innerModel.getProperties() != null) {
-                props.addAll(innerModel.getProperties().values());
+                // Schema.getName() is @JsonIgnore, so any prior JSON-based clone of innerModel
+                // (e.g. AnnotationsUtils.clone) leaves nested property schemas with a null name
+                // while the properties-map key still carries the correct name. Restore from the
+                // map key so the eventual `modelProps.put(prop.getName(), prop)` does not insert
+                // a null key (see swagger-api/swagger-core#5126).
+                for (Map.Entry<String, Schema> entry : ((Map<String, Schema>) innerModel.getProperties()).entrySet()) {
+                    Schema prop = entry.getValue();
+                    if (prop.getName() == null) {
+                        prop.setName(entry.getKey());
+                    }
+                    props.add(prop);
+                }
                 if (innerModel.getRequired() != null) {
                     requiredProps.addAll(innerModel.getRequired());
                 }
@@ -1475,10 +1485,14 @@ private void handleUnwrapped(List<Schema> props, Schema innerModel, String prefi
                 suffix = "";
             }
             if (innerModel.getProperties() != null) {
-                for (Schema prop : (Collection<Schema>) innerModel.getProperties().values()) {
+                for (Map.Entry<String, Schema> entry : ((Map<String, Schema>) innerModel.getProperties()).entrySet()) {
+                    Schema prop = entry.getValue();
                     try {
                         Schema clonedProp = Json.mapper().readValue(Json.pretty(prop), Schema.class);
-                        clonedProp.setName(prefix + prop.getName() + suffix);
+                        // Fall back to the map key when the prop's transient name has been lost
+                        // by a prior clone (Schema.getName() is @JsonIgnore).
+                        String baseName = prop.getName() != null ? prop.getName() : entry.getKey();
+                        clonedProp.setName(prefix + baseName + suffix);
                         props.add(clonedProp);
                     } catch (IOException e) {
                         LOGGER.error("Exception cloning property", e);
diff --git a/modules/swagger-core/src/test/java/io/swagger/v3/core/resolving/Ticket5126Test.java b/modules/swagger-core/src/test/java/io/swagger/v3/core/resolving/Ticket5126Test.java
new file mode 100644
index 0000000000..f9eca6c955
--- /dev/null
+++ b/modules/swagger-core/src/test/java/io/swagger/v3/core/resolving/Ticket5126Test.java
@@ -0,0 +1,95 @@
+package io.swagger.v3.core.resolving;
+
+import com.fasterxml.jackson.databind.ObjectMapper;
+import io.swagger.v3.core.jackson.ModelResolver;
+import io.swagger.v3.oas.models.media.Schema;
+import org.testng.annotations.BeforeMethod;
+import org.testng.annotations.Test;
+
+import java.lang.reflect.Method;
+import java.util.ArrayList;
+import java.util.LinkedHashMap;
+import java.util.LinkedHashSet;
+import java.util.List;
+import java.util.Map;
+
+import static org.testng.Assert.assertEquals;
+import static org.testng.Assert.assertNotNull;
+
+/**
+ * Regression test for <a href="https://github.com/swagger-api/swagger-core/issues/5126">#5126</a>.
+ *
+ * <p>When an inner model's nested property schemas have a {@code null} name — which happens
+ * naturally after {@code AnnotationsUtils.clone} round-trips the schema through JSON, because
+ * {@code Schema.getName()} is {@code @JsonIgnore} — {@link ModelResolver#handleUnwrapped} would
+ * forward those schemas into the outer model's properties list as-is. The subsequent
+ * {@code modelProps.put(prop.getName(), prop)} then inserts a {@code null} key into the outer's
+ * properties map, which later causes Jackson to fail serializing the OpenAPI document with
+ * {@code JsonMappingException: Null key for a Map not allowed in JSON}.
+ *
+ * <p>This test invokes {@code handleUnwrapped} directly with a hand-crafted inner model whose
+ * property schemas have a {@code null} name (simulating the post-clone state) and verifies that
+ * the resulting property names come from the inner properties-map keys, not from the transient
+ * {@code Schema.name} field.
+ */
+public class Ticket5126Test extends SwaggerTestBase {
+
+    private ModelResolver modelResolver;
+    private Method handleUnwrapped;
+
+    @BeforeMethod
+    public void setup() throws Exception {
+        modelResolver = new ModelResolver(new ObjectMapper());
+        handleUnwrapped = ModelResolver.class.getDeclaredMethod(
+                "handleUnwrapped", List.class, Schema.class, String.class, String.class, List.class);
+        handleUnwrapped.setAccessible(true);
+    }
+
+    @Test
+    public void noPrefixOrSuffix_restoresNameFromInnerPropertiesMapKey() throws Exception {
+        Schema<?> nameSchema = new Schema<>().type("string");      // name field unset
+        Schema<?> countSchema = new Schema<>().type("integer");    // name field unset
+        Schema<Object> inner = new Schema<>();
+        Map<String, Schema> innerProps = new LinkedHashMap<>();
+        innerProps.put("name", nameSchema);
+        innerProps.put("count", countSchema);
+        inner.setProperties(innerProps);
+
+        List<Schema> props = new ArrayList<>();
+        List<String> requiredProps = new ArrayList<>();
+        handleUnwrapped.invoke(modelResolver, props, inner, "", "", requiredProps);
+
+        assertEquals(props.size(), 2);
+        LinkedHashSet<String> names = new LinkedHashSet<>();
+        for (Schema p : props) {
+            assertNotNull(p.getName(),
+                    "Each unwrapped property must carry a non-null name to avoid null keys in the outer's properties map");
+            names.add(p.getName());
+        }
+        assertEquals(names, new LinkedHashSet<>(java.util.Arrays.asList("name", "count")));
+    }
+
+    @Test
+    public void withPrefixAndSuffix_combinesMapKeyWhenInnerNameIsNull() throws Exception {
+        Schema<?> nameSchema = new Schema<>().type("string");
+        Schema<?> countSchema = new Schema<>().type("integer");
+        Schema<Object> inner = new Schema<>();
+        Map<String, Schema> innerProps = new LinkedHashMap<>();
+        innerProps.put("name", nameSchema);
+        innerProps.put("count", countSchema);
+        inner.setProperties(innerProps);
+
+        List<Schema> props = new ArrayList<>();
+        List<String> requiredProps = new ArrayList<>();
+        handleUnwrapped.invoke(modelResolver, props, inner, "p_", "_s", requiredProps);
+
+        assertEquals(props.size(), 2);
+        LinkedHashSet<String> names = new LinkedHashSet<>();
+        for (Schema p : props) {
+            assertNotNull(p.getName(),
+                    "Prefixed/suffixed names must be derived from the inner properties-map key when the original Schema.name has been lost");
+            names.add(p.getName());
+        }
+        assertEquals(names, new LinkedHashSet<>(java.util.Arrays.asList("p_name_s", "p_count_s")));
+    }
+}