diff --git a/docs/src/APIs/datalayouts_api.md b/docs/src/APIs/datalayouts_api.md
index c81b3e9257..acea4306dc 100644
--- a/docs/src/APIs/datalayouts_api.md
+++ b/docs/src/APIs/datalayouts_api.md
@@ -19,3 +19,14 @@ DataLayouts.IJHF
 DataLayouts.VIHF
 DataLayouts.VIJHF
 ```
+
+## Internals
+
+```@docs
+check_basetype
+is_valid_basetype
+first_invalid_primitive_type
+replace_basetype
+parent_array_type
+promote_parent_array_type
+```
diff --git a/src/DataLayouts/struct.jl b/src/DataLayouts/struct.jl
index f6de344c95..26e612d611 100644
--- a/src/DataLayouts/struct.jl
+++ b/src/DataLayouts/struct.jl
@@ -24,6 +24,21 @@ is_valid_basetype(::Type{T}, ::Type{<:T}) where {T} = true
 is_valid_basetype(::Type{T}, ::Type{S}, Ss...) where {T, S} =
     is_valid_basetype(T, S) && is_valid_basetype(T, Ss...)
 
+"""
+    first_invalid_primitive_type(::Type{T}, ::Type{S})
+
+Returns the first primitive subtype of `S` that cannot be represented using
+base type `T`, or `nothing` if `T` is a valid base type for `S`.
+"""
+first_invalid_primitive_type(::Type{T}, ::Type{S}) where {T, S} = 
+    is_valid_basetype(T, S) ? nothing :
+    Base.isprimitivetype(S) ? S :
+    first_invalid_primitive_type(T, fieldtypes(S)...)
+first_invalid_primitive_type(::Type{T}, ::Type{S}, Ss...) where {T, S} =
+    isnothing(first_invalid_primitive_type(T, S)) ?
+        first_invalid_primitive_type(T, Ss...) :
+        first_invalid_primitive_type(T, S)
+
 """
     check_basetype(::Type{T}, ::Type{S})
 
@@ -39,7 +54,9 @@ function check_basetype(::Type{T}, ::Type{S}) where {T, S}
             estr = "Struct type $S has indeterminate size"
             :(error($estr))
         elseif !is_valid_basetype(T, S)
-            estr = "Struct type $S cannot be represented using base type $T"
+            P = first_invalid_primitive_type(T, S)
+            estr = "Struct type $S contains subtype $P, \
+                    which cannot be represented using base type $T"
             :(error($estr))
         else
             :(return nothing)
@@ -47,8 +64,10 @@ function check_basetype(::Type{T}, ::Type{S}) where {T, S}
     else
         isbitstype(T) || error("Base type $T has indeterminate size")
         isbitstype(S) || error("Struct type $S has indeterminate size")
+        P = first_invalid_primitive_type(T, S)
         is_valid_basetype(T, S) ||
-            error("Struct type $S cannot be represented using base type $T")
+            error("Struct type $S contains subtype $P, \
+                   which cannot be represented using base type $T")
         return nothing
     end
 end