diff --git a/CHANGELOG.md b/CHANGELOG.md
index 3d1f7bed..77b90ffb 100644
--- a/CHANGELOG.md
+++ b/CHANGELOG.md
@@ -6,7 +6,9 @@
 
 ### Bug Fixes
 
-_None_
+* The context enrich function won't overwrite existing values in the `env` and `param` variables.  
+  [David Jennes](https://github.com/djbe)
+  [#29](https://github.com/SwiftGen/SwiftGenKit/issues/29)
 
 ### Breaking Changes
 
diff --git a/Sources/Context.swift b/Sources/Context.swift
index b26d34ad..8791e04c 100644
--- a/Sources/Context.swift
+++ b/Sources/Context.swift
@@ -9,8 +9,8 @@
 import Foundation
 
 public enum StencilContext {
-  public static let environment = "env"
-  public static let parameters = "param"
+  public static let environmentKey = "env"
+  public static let parametersKey = "param"
 
   /**
    Enriches a stencil context with parsed parameters and environment variables
@@ -24,9 +24,19 @@ public enum StencilContext {
                             environment: [String: String] = ProcessInfo().environment) throws -> [String: Any] {
     var context = context
 
-    context[StencilContext.environment] = environment
-    context[StencilContext.parameters] = try Parameters.parse(items: parameters)
+    context[environmentKey] = merge(context[environmentKey], with: environment)
+    context[parametersKey] = merge(context[parametersKey], with: try Parameters.parse(items: parameters))
 
     return context
   }
+
+  private static func merge(_ lhs: Any?, with rhs: [String: Any]) -> [String: Any] {
+    var result = lhs as? [String: Any] ?? [:]
+
+    for (key, value) in rhs {
+      result[key] = value
+    }
+
+    return result
+  }
 }
diff --git a/Tests/StencilSwiftKitTests/ContextTests.swift b/Tests/StencilSwiftKitTests/ContextTests.swift
index 82b21953..a80cf65c 100644
--- a/Tests/StencilSwiftKitTests/ContextTests.swift
+++ b/Tests/StencilSwiftKitTests/ContextTests.swift
@@ -18,13 +18,13 @@ class ContextTests: XCTestCase {
                                            environment: ["PATH": "foo:bar:baz"])
     XCTAssertEqual(result.count, 2, "2 items have been added")
 
-    guard let env = result[StencilContext.environment] as? [String: Any] else {
+    guard let env = result[StencilContext.environmentKey] as? [String: Any] else {
       XCTFail("`env` should be a dictionary")
       return
     }
     XCTAssertEqual(env["PATH"] as? String, "foo:bar:baz")
 
-    guard let params = result[StencilContext.parameters] as? [String: Any] else {
+    guard let params = result[StencilContext.parametersKey] as? [String: Any] else {
       XCTFail("`param` should be a dictionary")
       return
     }
@@ -41,13 +41,13 @@ class ContextTests: XCTestCase {
     XCTAssertEqual(result["foo"] as? String, "bar")
     XCTAssertEqual(result["hello"] as? Bool, true)
 
-    guard let env = result[StencilContext.environment] as? [String: Any] else {
+    guard let env = result[StencilContext.environmentKey] as? [String: Any] else {
       XCTFail("`env` should be a dictionary")
       return
     }
     XCTAssertEqual(env["PATH"] as? String, "foo:bar:baz")
 
-    guard let params = result[StencilContext.parameters] as? [String: Any] else {
+    guard let params = result[StencilContext.parametersKey] as? [String: Any] else {
       XCTFail("`param` should be a dictionary")
       return
     }
@@ -62,13 +62,13 @@ class ContextTests: XCTestCase {
                                            environment: ["PATH": "foo:bar:baz"])
     XCTAssertEqual(result.count, 2, "2 items have been added")
 
-    guard let env = result[StencilContext.environment] as? [String: Any] else {
+    guard let env = result[StencilContext.environmentKey] as? [String: Any] else {
       XCTFail("`env` should be a dictionary")
       return
     }
     XCTAssertEqual(env["PATH"] as? String, "foo:bar:baz")
 
-    guard let params = result[StencilContext.parameters] as? [String: Any] else {
+    guard let params = result[StencilContext.parametersKey] as? [String: Any] else {
       XCTFail("`param` should be a dictionary")
       return
     }