diff --git a/src/crystal/system/win32/process.cr b/src/crystal/system/win32/process.cr
index fa0754116d2e..46e1d25ee9aa 100644
--- a/src/crystal/system/win32/process.cr
+++ b/src/crystal/system/win32/process.cr
@@ -256,8 +256,9 @@ struct Crystal::System::Process
     process_info = LibC::PROCESS_INFORMATION.new
 
     if command_args.is_a?(String)
+      raise ArgumentError.new("Command line contains newline") if command_args.includes?('\n')
       result = LibC.CreateProcessW(
-        CMD_EXE, System.to_wstr(%(/c "#{command_args}")), nil, nil, true, LibC::CREATE_UNICODE_ENVIRONMENT,
+        CMD_EXE, System.to_wstr(%(/s /c "#{command_args}")), nil, nil, true, LibC::CREATE_UNICODE_ENVIRONMENT,
         make_env_block(env, clear_env), chdir.try { |str| System.to_wstr(str) } || Pointer(UInt16).null,
         pointerof(startup_info), pointerof(process_info)
       )
diff --git a/src/process.cr b/src/process.cr
index 8509ea730341..31718733b796 100644
--- a/src/process.cr
+++ b/src/process.cr
@@ -234,7 +234,7 @@ class Process
   # * On Windows this is implemented by passing the string as-is to `%COMSPEC%`,
   #   and passing *args* is not supported. The user is responsible for correctly
   #   quoting the command string, as `Process.quote_windows` does not handle
-  #   this.
+  #   this. Additionally, *command* cannot contain the newline character (`\n`).
   #
   # Raises `IO::Error` if executing the command fails (for example if the executable doesn't exist).
   def initialize(command : String, args = nil, env : Env = nil, clear_env : Bool = false, shell : Bool = false,