diff --git a/modules/openapi-generator/src/main/resources/kotlin-client/libraries/jvm-ktor/api.mustache b/modules/openapi-generator/src/main/resources/kotlin-client/libraries/jvm-ktor/api.mustache
index de8c6a874471..c381f3a38749 100644
--- a/modules/openapi-generator/src/main/resources/kotlin-client/libraries/jvm-ktor/api.mustache
+++ b/modules/openapi-generator/src/main/resources/kotlin-client/libraries/jvm-ktor/api.mustache
@@ -69,10 +69,10 @@ import com.fasterxml.jackson.databind.ObjectMapper
                         {{^isArray}}
                         {{^isString}}
                         {{^isNumber}}
-                        {{{paramName}}}?.apply { append("{{{baseName}}}", {{{paramName}}}.toString()) }
+                        {{{paramName}}}?.apply { append("{{{baseName}}}", {{{paramName}}}?.toString()) }
                         {{/isNumber}}
                         {{#isNumber}}
-                        {{{paramName}}}?.apply { append("{{{baseName}}}", {{{paramName}}}) }
+                        {{{paramName}}}?.apply { append("{{{baseName}}}", {{{paramName}}}?.toString()) }
                         {{/isNumber}}
                         {{/isString}}
                         {{#isString}}
@@ -98,10 +98,10 @@ import com.fasterxml.jackson.databind.ObjectMapper
                         {{^isArray}}
                         {{^isString}}
                         {{^isNumber}}
-                        {{{paramName}}}?.apply { it.append("{{{baseName}}}", {{{paramName}}}.toString()) }
+                        {{{paramName}}}?.apply { it.append("{{{baseName}}}", {{{paramName}}}?.toString()) }
                         {{/isNumber}}
                         {{#isNumber}}
-                        {{{paramName}}}?.apply { it.append("{{{baseName}}}", {{{paramName}}}) }
+                        {{{paramName}}}?.apply { it.append("{{{baseName}}}", {{{paramName}}}?.toString()) }
                         {{/isNumber}}
                         {{/isString}}
                         {{#isString}}