diff --git a/src/SurveyDesign.jl b/src/SurveyDesign.jl
index 71ee1356..fc8030f1 100644
--- a/src/SurveyDesign.jl
+++ b/src/SurveyDesign.jl
@@ -70,7 +70,7 @@ struct SurveyDesign <: AbstractSurveyDesign
         end
         ## Single stage approximation
         if typeof(clusters) <: Vector{Symbol}
-            @warn As part of single-stage approximation, only the first stage cluster ID is retained. 
+            @warn "As part of single-stage approximation, only the first stage cluster ID is retained." 
             cluster = first(clusters)
         end
         if typeof(clusters) <: Symbol