diff --git a/modules/openapi-generator/src/main/java/org/openapitools/codegen/languages/CSharpNetCoreClientCodegen.java b/modules/openapi-generator/src/main/java/org/openapitools/codegen/languages/CSharpNetCoreClientCodegen.java
index 6bdde984774..b2136e16593 100644
--- a/modules/openapi-generator/src/main/java/org/openapitools/codegen/languages/CSharpNetCoreClientCodegen.java
+++ b/modules/openapi-generator/src/main/java/org/openapitools/codegen/languages/CSharpNetCoreClientCodegen.java
@@ -31,6 +31,7 @@ import java.io.File;
 import java.util.*;
 import java.util.concurrent.atomic.AtomicReference;
 import java.util.function.Consumer;
+import java.util.stream.Collectors;
 
 import static org.apache.commons.lang3.StringUtils.isEmpty;
 import static org.openapitools.codegen.utils.StringUtils.camelize;
@@ -547,7 +548,6 @@ public class CSharpNetCoreClientCodegen extends AbstractCSharpCodegen {
             additionalProperties.put(CodegenConstants.OPTIONAL_EMIT_DEFAULT_VALUES, optionalEmitDefaultValuesFlag);
         }
 
-
         if (additionalProperties.containsKey(CodegenConstants.MODEL_PROPERTY_NAMING)) {
             setModelPropertyNaming((String) additionalProperties.get(CodegenConstants.MODEL_PROPERTY_NAMING));
         }
@@ -561,13 +561,24 @@ public class CSharpNetCoreClientCodegen extends AbstractCSharpCodegen {
         clientPackage = "Client";
 
         String framework = (String) additionalProperties.getOrDefault(CodegenConstants.DOTNET_FRAMEWORK, defaultFramework.name);
+        boolean strategyMatched = false;
         FrameworkStrategy strategy = defaultFramework;
         for (FrameworkStrategy frameworkStrategy : frameworkStrategies) {
             if (framework.equals(frameworkStrategy.name)) {
                 strategy = frameworkStrategy;
+                strategyMatched = true;
             }
         }
 
+        // throws exception if the input targetFramework is invalid
+        if (strategyMatched == false) {
+            throw new IllegalArgumentException("Invalid .NET framework version: " +
+                    framework + ". List of supported versions: " +
+                    frameworkStrategies.stream()
+                    .map(p -> p.name)
+                    .collect(Collectors.joining(", ")));
+        }
+
         strategy.configureAdditionalProperties(additionalProperties);
 
         setTargetFrameworkNuget(strategy.getNugetFrameworkIdentifier());
@@ -710,7 +721,11 @@ public class CSharpNetCoreClientCodegen extends AbstractCSharpCodegen {
 
     public void setTargetFramework(String dotnetFramework) {
         if (!frameworks.containsKey(dotnetFramework)) {
-            LOGGER.warn("Invalid .NET framework version, defaulting to " + this.targetFramework);
+            throw new IllegalArgumentException("Invalid .NET framework version: " +
+                    dotnetFramework + ". List of supported versions: " +
+                    frameworkStrategies.stream()
+                    .map(p -> p.name)
+                    .collect(Collectors.joining(", ")));
         } else {
             this.targetFramework = dotnetFramework;
         }