diff --git a/src/named_systems2.jl b/src/named_systems2.jl
index ebe0e210..2e71041e 100644
--- a/src/named_systems2.jl
+++ b/src/named_systems2.jl
@@ -17,6 +17,21 @@ function check_unique(vals, s, msg=""; throw = true)
     return true
 end
 
+_check_external_names(::Any, _allnames, _argname, _label, _hint) = nothing
+function _check_external_names(names::AbstractVector{Symbol}, allnames, argname, label, hint)
+    snames = string.(allnames)
+    for name in names
+        name in allnames && continue
+        matches = allnames[findall(startswith(string(name)), snames)]
+        if isempty(matches)
+            error("The name $name in `$argname` was not found among the $label. Available are $(allnames)$hint.")
+        elseif length(matches) > 1
+            error("The name $name in `$argname` is ambiguous: it prefix-matches multiple $label $matches. Use the exact name to disambiguate$hint.")
+        end
+    end
+    return nothing
+end
+
 function check_all_unique(s1, s2; throw=true)
     valx = check_unique([getproperty(s1, :x); getproperty(s2, :x)], "x"; throw)
     check_unique([getproperty(s1, :u); getproperty(s2, :u)], "u"; throw=true)
@@ -675,6 +690,9 @@ function connect(systems; u1::Vector{Symbol}, y1::Vector{Symbol}, external_input
 
     check_unique(full.y, "system outputs")
 
+    _check_external_names(w1, full.u, "external_inputs", "system inputs", "; insert a `splitter(name, n)` to expose a fresh external signal connected to multiple ports")
+    _check_external_names(z1, full.y, "external_outputs", "system outputs", "")
+
     if verbose
         leftover_inputs = setdiff(full.u, [u1; w1])
         isempty(leftover_inputs) || @warn("The following inputs were unconnected $leftover_inputs, ignore this warning if you rely on prefix matching. Turn off this warning by passing `verbose = false`.")
diff --git a/test/test_named_systems2.jl b/test/test_named_systems2.jl
index b3e1cdb5..fc55b537 100644
--- a/test/test_named_systems2.jl
+++ b/test/test_named_systems2.jl
@@ -398,6 +398,42 @@ nss = named_ss(tf(1, [1,1]))
 @test nss isa NamedStateSpace
 
 
+## Issue #114 — `connect` must reject ambiguous or unknown
+##  `external_inputs`/`external_outputs` names. Previously such names silently
+##  prefix-matched multiple internal ports and produced wrong externals.
+sys_y1 = named_ss(ss(-1.0), u=:yinu, y=:youta)
+sys_y2 = named_ss(ss( 1.0), u=:yiny, y=:youtb)
+
+# :y is supposed to be external but prefix-matches BOTH :yinu and :yiny
+@test_throws "external_inputs" connect(
+    [sys_y1, sys_y2],
+    Pair{Symbol,Symbol}[];
+    external_inputs = [:y],
+    external_outputs = [:youta, :youtb],
+)
+
+# Symmetric: :yout prefix-matches both :youta and :youtb
+@test_throws "external_outputs" connect(
+    [sys_y1, sys_y2],
+    Pair{Symbol,Symbol}[];
+    external_inputs = [:yinu, :yiny],
+    external_outputs = [:yout],
+)
+
+# Unknown name (no exact, no prefix) errors too
+@test_throws "external_inputs" connect(
+    [sys_y1, sys_y2],
+    Pair{Symbol,Symbol}[];
+    external_inputs = [:totally_bogus],
+    external_outputs = [:youta, :youtb],
+)
+
+# Single-prefix match (e.g. `:x` -> `Symbol("x(t)")`) must still work — used by LQG tests
+sys_a = named_ss(ssrand(1, 1, 1), u=Symbol("x(t)"), y=Symbol("y(t)"), x=:xa)
+sys_b = named_ss(ssrand(1, 1, 1), u=:u_b,            y=:y_b,           x=:xb)
+@test connect([sys_a, sys_b], [Symbol("y(t)") => :u_b];
+              external_inputs = [:x], external_outputs = [:y_b]) isa NamedStateSpace
+
 
 ## Test where one external input goes to several inputs with the same name, with and without using a splitter
 s1 = ssrand(1,2,2)