diff --git a/src/QuestBase.jl b/src/QuestBase.jl
index 8a80959..db05c1b 100644
--- a/src/QuestBase.jl
+++ b/src/QuestBase.jl
@@ -27,7 +27,6 @@ using Symbolics:
     Equation,
     Differential,
     arguments,
-    substitute,
     term,
     expand,
     operation,
diff --git a/src/Symbolics/Symbolics_utils.jl b/src/Symbolics/Symbolics_utils.jl
index 495265f..09ecefb 100644
--- a/src/Symbolics/Symbolics_utils.jl
+++ b/src/Symbolics/Symbolics_utils.jl
@@ -63,8 +63,26 @@ Perform substitutions in `rules` on `x`.
 `Symbolics.substitute_in_deriv`.
 """
 Subtype = Union{Num,Equation,BasicSymbolic}
+
+# SymbolicUtils 4 / Symbolics 7: substitute() does not recurse into call arguments
+# (e.g. it leaves x(t) unchanged when substituting t => T). Walk the expression and
+# rewrite matching nodes ourselves.
+function _deep_substitute(e::BasicSymbolic, unwrap_rules::Dict)
+    # SU 4's default filter blocks recursion into the arguments of callable-symbolic
+    # terms (e.g. `x(t)`). Override with an always-true filter so substitutions like
+    # `t => T` reach inside `x(t)`. Compound keys still match before recursion, and
+    # SU does not re-walk the replacement, so self-referential rules don't loop.
+    return SymbolicUtils.substitute(e, unwrap_rules; filterer = _ -> true)
+end
+_deep_substitute(x::Num, ur::Dict) = wrap(_deep_substitute(unwrap(x), ur))
+function _deep_substitute(eq::Equation, ur::Dict)
+    return Equation(_deep_substitute(unwrap(eq.lhs), ur), _deep_substitute(unwrap(eq.rhs), ur))
+end
+_deep_substitute(x, ::Dict) = x
+
 function substitute_all(x::Subtype, rules::Dict; include_derivatives=true)
-    result = substitute(x, rules)
+    unwrap_rules = Dict(unwrap(k) => unwrap(v) for (k, v) in rules)
+    result = _deep_substitute(x, unwrap_rules)
     if include_derivatives
         result = Symbolics.substitute_in_deriv(result, rules)
     end
diff --git a/src/Symbolics/fourier.jl b/src/Symbolics/fourier.jl
index 233baa9..c21f0f5 100644
--- a/src/Symbolics/fourier.jl
+++ b/src/Symbolics/fourier.jl
@@ -37,8 +37,9 @@ is_trig(f::Num) = is_trig(unwrap(f))
 is_trig(f) = false
 function is_trig(f::BasicSymbolic)
     f = ispow(f) ? arguments(f)[1] : f
-    isterm(f) && operation(f) ∈ [cos, sin] && return true
-    return false
+    isterm(f) || return false
+    op = operation(f)
+    return op === cos || op === sin
 end
 
 """
diff --git a/src/utils.jl b/src/utils.jl
index fe063ea..76bfc82 100644
--- a/src/utils.jl
+++ b/src/utils.jl
@@ -37,4 +37,8 @@ end
 is_identity(A::Matrix{Num}) = (@eqsym A == Matrix{Num}(LinearAlgebra.I, size(A)...))
 hasnan(x::Matrix{Num}) = any(my_isnan, unwrap.(x))
 my_isnan(x) = isnan(x)
-my_isnan(x::BasicSymbolic) = false
+function my_isnan(x::BasicSymbolic)
+    SymbolicUtils.isconst(x) || return false
+    v = Symbolics.value(x)
+    return v isa Number && isnan(v)
+end